Exhibit 99.(a)(1)

THIS DOCUMENT IS IMPORTANT AND REQUIRES YOUR IMMEDIATE ATTENTION. If you are in any doubt as to the action you should take, you are recommended to seek your own personal financial advice from your stockbroker, bank manager, solicitor, accountant or other independent financial adviser authorised under the Financial Services and Markets Act 2000 if you are resident in the United Kingdom or, if not, from another appropriately authorised independent financial adviser.

This document should be read in conjunction with the accompanying Acceptance Forms, which form part of this document.

If you have sold or otherwise transferred all of your Celltech Shares or Celltech ADSs (other than pursuant to the Offer), please send this document, together with the accompanying documents and the reply-paid envelope as soon as possible to the purchaser or transferee, or to the stockbroker, bank or other agent through whom the sale or transfer was effected, for transmission to the purchaser or transferee. If you have sold or otherwise transferred only part of your holding of Celltech Shares or Celltech ADSs, you should retain these documents. However, the foregoing documents must not be forwarded or transmitted in or into Australia, Belgium, Canada or Japan or any jurisdiction where to do so would constitute a violation of the relevant laws in that jurisdiction.

Recommended Cash Offer
by
Lazard & Co., Limited
on behalf of
UCB S.A.
and (in the United States) by UCB S.A. itself
for
Celltech Group plc

YOUR ATTENTION IS DRAWN TO THE LETTER FROM THE CHAIRMAN OF CELLTECH, WHICH IS SET OUT IN PART I OF THIS DOCUMENT AND WHICH CONTAINS THE RECOMMENDATION OF THE CELLTECH BOARD TO ACCEPT THE OFFER.

IF YOU ARE A HOLDER OF CELLTECH SHARES AND WISH TO ACCEPT THE OFFER, THE ACCOMPANYING FORM OF ACCEPTANCE SHOULD BE COMPLETED, SIGNED AND RETURNED, WHETHER OR NOT YOUR CELLTECH SHARES ARE HELD IN CREST, IN ACCORDANCE WITH THE INSTRUCTIONS PRINTED THEREON, AS SOON AS POSSIBLE AND, IN ANY EVENT, SO AS TO BE RECEIVED BY CAPITA IRG PLC, CORPORATE ACTIONS, PO BOX 166, THE REGISTRY, 34 BECKENHAM ROAD, BECKENHAM, KENT BR3 4TH NOT LATER THAN 3.00 P.M. (LONDON TIME), 10.00 A.M. (NEW YORK CITY TIME), ON 17 JUNE 2004.

THE PROCEDURE FOR ACCEPTING THE OFFER IS SET OUT IN PARAGRAPH 18 OF THE LETTER FROM LAZARD SET OUT IN PART II OF THIS DOCUMENT AND IN THE FORM OF ACCEPTANCE.

IF YOU ARE A HOLDER OF CELLTECH ADSs AND WISH TO ACCEPT THE OFFER, YOU SHOULD FOLLOW THE INSTRUCTIONS SET OUT IN PARAGRAPH 18(c) OF THE LETTER FROM LAZARD SET OUT IN PART II OF THIS DOCUMENT AND IN THE LETTER OF TRANSMITTAL.

        Lazard is acting for UCB and no one else in connection with the Offer and will not be responsible to anyone other than UCB for providing the protections afforded to clients of Lazard or for providing advice in relation to the Offer or the contents of this document.

        Each of Morgan Stanley and JPMorgan is acting for Celltech and no one else in connection with the Offer and will not be responsible to anyone other than Celltech for providing the protections afforded to clients of Morgan Stanley or JPMorgan (as the case may be) or for providing advice in relation to the Offer or the contents of this document.

        The Offer in the United States is made solely by UCB and neither Lazard nor any of its affiliates is making the Offer in the United States.

        Unless otherwise determined by UCB and permitted by applicable law and regulation, the Offer (including the Loan Note Alternative) will not be made, directly or indirectly, in or into a Restricted Jurisdiction and, subject to certain exemptions, the Offer will not be capable of acceptance from or within a Restricted Jurisdiction. Accordingly, copies of this document and the Acceptance Forms and any other accompanying document are not being, and must not be, directly or indirectly, mailed, forwarded, transmitted, sent or otherwise distributed or sent in, into or from a Restricted Jurisdiction and persons receiving this document, the Acceptance Forms and any other accompanying document (including custodians, nominees and trustees) must not mail, forward, transmit, send or otherwise distribute them in, into or from such jurisdictions as doing so may invalidate any purported acceptance of the Offer. The availability of the Offer to persons who are not resident in the United Kingdom or the United States may be affected by the laws of their relevant jurisdiction. Such persons should inform themselves about and observe any applicable legal or regulatory requirements of their jurisdiction.

        The Loan Notes which may be issued pursuant to the Loan Note Alternative have not been, and will not be, listed on any stock exchange and have not been, and will not be, registered under the Securities Act or under any relevant laws of any state or other jurisdiction of the United States, nor have clearances been, nor will they be, obtained from the securities commission or similar authority of any province or territory of Canada and no prospectus has been, or will be, filed, or registration made, under any securities law of any province or territory of Canada, nor has a prospectus in relation to the Loan Notes been, nor will one be, lodged with, or registered by, the Australian Securities and Investments Commission, nor have any steps been taken, nor will any steps be taken, to enable the Loan Notes to be offered in compliance with applicable securities laws of Japan or Belgium. Accordingly, unless an exemption under relevant securities laws is available, the Loan Notes may not be offered, sold, re-sold or delivered, directly or indirectly, in, into or from the United States or a Restricted Jurisdiction in which an offer of Loan Notes would constitute a violation of relevant laws or require registration of the Loan Notes, or to or for the account or benefit of any US person or resident of a Restricted Jurisdiction. Under the terms of the Offer, holders of Celltech Securities who are US persons or otherwise located in the United States are not eligible to receive Loan Notes.

        Any person (including nominees, trustees and custodians) who would, or otherwise intends to, or may have a legal or contractual obligation to, forward this document and/or any related document to any jurisdiction outside the United Kingdom or the United States, should read paragraph 6 of Part B of Appendix I to this document before taking any action.

i

TO ACCEPT THE OFFER

1.

Complete the Form of Acceptance in accordance with paragraph 18 of the letter from Lazard set out in Part II of this document (see pages 9 to 25). A step-by-step guide to completing the Form of Acceptance has been sent to you with this document.

2.

Return as soon as possible the completed Form of Acceptance (along with any appropriate documents of title, such as your share certificate(s)) using the enclosed reply-paid envelope and, in any event, so as to be received by Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent, BR3 4TH, no later than 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004.

3.

If you hold your Celltech Shares in CREST, you should follow the additional procedures set out in paragraph 18(b) of the letter from Lazard set out in Part II of this document (see pages 9 to 25).

SHAREHOLDER HELPLINE 0870 162 3118 (calls charged at national rate)                       
(800) 261 1054 (toll free, if telephoning in the US)
+44 208 639 2157 (if telephoning from other countries)

Open Monday to Friday, 9 a.m. to 5 p.m. (London time)

        For legal reasons, the Shareholder Helpline will only be able to provide information contained in this document and the Form(s) of Acceptance and will be unable to give advice on the merits of the Offer or to provide financial advice.

Acceptances of the Offer must be received by 3.00 p.m. (London time), 10.00 a.m. (New York City time) on 17 June 2004.

IF YOU ARE A HOLDER OF CELLTECH ADSs, TO ACCEPT THE OFFER:

1.

complete the Letter of Transmittal in accordance with paragraph 18(c) of the letter from Lazard; and

2.

return as soon as possible the completed Letter of Transmittal (along with any appropriate documents of title, such as your Celltech ADRs) to the Tender Agent, using the enclosed reply-paid envelope.

ii

Offer into the United States

If you are a resident of the United States, please read the following:

        The Offer is being made for securities of a UK company and, while the Offer is subject to UK and US disclosure requirements, US investors should be aware that this Offer Document has been prepared in accordance with a UK format and style, which differs from the US format and style. In particular, the appendices to this document contain information concerning the Offer required by UK and US disclosure requirements which may be material and which has not been summarised elsewhere in this document. In addition, the financial statements of UCB reproduced in this document have been prepared in accordance with generally accepted accounting principles in Belgium and the financial statements of Celltech reproduced in this document have been prepared in accordance with generally accepted accounting principles in the United Kingdom and thus neither set of financial statements may be comparable to financial statements of US companies or companies whose financial statements are prepared in accordance with generally accepted accounting principles in the United States. In addition, the settlement procedure with respect to the Offer will comply with the rules of the City Code, which differ from US domestic tender offer procedures in certain material respects, particularly with regard to the date of payment of consideration.

        Celltech is organised under the laws of England and Wales. UCB is organised under the laws of Belgium. Some or all of the officers and directors of Celltech and UCB, respectively, are residents of countries other than the United States. In addition, a significant portion of the assets of Celltech and UCB are located outside the United States. As a result, it may be difficult for US shareholders of Celltech to effect service of process within the United States upon Celltech or UCB or their respective officers or directors or to enforce against them a judgement of a US court predicated upon the federal or state securities laws of the United States.

        As part of the Offer, Celltech Shareholders who are eligible to do so may elect for the Loan Note Alternative. The effect of electing for the Loan Note Alternative will be to allow eligible Celltech Shareholders resident in the United Kingdom to defer any taxable gain arising on a disposal of their Celltech Shares until such time as the Loan Notes are transferred or redeemed. Such tax treatment is not available for the Loan Note Alternative under US federal income tax laws and the Loan Note Alternative is not available to Celltech Shareholders who are US persons or otherwise in the United States. The Loan Note Alternative is also not available to holders of Celltech ADSs.

        In accordance with the City Code, normal UK practice and Rule 14e-5 under the Exchange Act ("Rule 14e-5"), Lazard, Morgan Stanley and JPMorgan and/or their respective affiliates will continue to act as connected exempt market makers or connected exempt principal traders in Celltech Shares on the London Stock Exchange. Information regarding such activities which is required to be made public in the United Kingdom pursuant to the City Code is reported to a Regulatory Information Service of the UK Listing Authority. This information will also be made available to US holders of Celltech Shares and Celltech ADSs, if such holders contact the UK or US Information Agent on the Helpline.

        In addition, in accordance with normal UK practice and pursuant to exemptive relief granted by the SEC from Rule 14e-5, UCB and its nominees or brokers (acting as agents) may make certain purchases of, or arrangements to purchase, Celltech Shares outside the United States during the period in which the Offer remains open for acceptance. In accordance with the requirements of Rule 14e-5 and with the exemptive relief granted by the SEC, such purchases, or arrangements to purchase, must comply with applicable UK rules, including the City Code, the rules of the UK Listing Authority and the rules of the London Stock Exchange. This information will be disclosed in the United States through amendments to UCB's Tender Offer Statement on Schedule TO on file with the SEC to the extent that such information is made public in the United Kingdom pursuant to the City Code. Free copies of the Tender Offer Statement are available on the SEC's website at http://www.sec.gov.

        This Offer Document includes "forward-looking statements" relating to the Offer, UCB and Celltech that are subject to known and unknown risks and uncertainties, many of which are outside of UCB's and Celltech's control and are difficult to predict and that may cause actual results to differ materially from any future results expressed or implied by such forward-looking statements. In this Offer Document, the words "anticipates," "believes," "estimates," "seeks," "expects," "plans," "intends" and similar expressions, as they relate to UCB or its management, are intended to identify forward-looking statements. Important factors that could cause actual results to differ materially from such expectations include, without limitation: the inability to obtain necessary regulatory approvals in the context of the Offer or to obtain

iii

them on acceptable terms; the inability to integrate successfully Celltech within UCB or to realise synergies from such integration following the acquisition; costs related to the acquisition of Celltech; the economic environment of the industries in which UCB and Celltech operate; costs associated with research and development; changes in the prospects for products in the pipeline or under development by UCB or Celltech; dependence on the existing management of UCB and Celltech; changes or uncertainties in UK or US federal or state tax laws or the administration of such laws; changes or uncertainties in the laws or regulations applicable to the markets in which UCB and Celltech operate, including those of the Federal Drug Administration in the United States; and other factors detailed in Celltech's filings with the SEC.

        All subsequent written and oral forward-looking statements attributable to UCB or Celltech or persons acting on behalf of either of them are expressly qualified in their entirety by the cautionary statements above. The forward-looking statements included herein are made only as of the date of this Offer Document. Neither UCB nor Celltech intend, or undertake any obligation, to update these forward-looking statements.

        Any person who, alone or acting together with any other person(s) pursuant to an agreement or any understanding (whether formal or informal) to acquire or control securities of Celltech, owns or controls, or becomes the owner or controller, directly or indirectly, of one per cent. or more of the issued share capital of Celltech is generally required under the provisions of Rule 8 of the City Code to notify a Regulatory Information Service and the Panel of every dealing in such securities during the Offer Period by not later than 12.00 noon (London time) on the business day following the date of the relevant transaction. Please consult your financial adviser immediately if you believe this Rule may be applicable to you.

        In the United States, the Offer is made solely by UCB, and neither Lazard nor any of its affiliates is making the Offer in the United States.

        Lazard Frères & Co. LLC is acting as dealer manager of the Offer in the United States.

iv

CONTENTS

v

FREQUENTLY ASKED QUESTIONS

        The following are some of the questions you, as a Celltech Shareholder and/or holder of Celltech ADSs, may have and answers to those questions. You are advised to read carefully the remainder of this document and the accompanying Form of Acceptance (in relation to Celltech Shares) or Letter of Transmittal (in relation to Celltech ADSs).

1      Who is offering to buy my shares?

The Offer is being made by UCB, a company organised under the laws of Belgium, and (outside the United States) by Lazard on its behalf. The main listing of UCB's ordinary shares is on the First Market of Euronext Brussels.

UCB is headquartered in Brussels, Belgium and is a pharmaceutical and specialty chemical company that does business through its approximately 120 subsidiaries in Europe, the Americas and Asia. UCB's businesses are focused in two sectors, UCB Pharma and Surface Specialties.

2      What are the classes and amounts of Celltech Securities sought in the Offer?

UCB is seeking to acquire all of the issued and to be issued Celltech Securities, comprised of:

•

Celltech Shares listed on the London Stock Exchange; and



•

Celltech ADSs listed on the New York Stock Exchange.

3      What will I receive in exchange for my Celltech Securities?

UCB is offering to pay:

4      How do I accept the Offer?

If you are a holder of Celltech Shares, to accept the Offer, you must deliver a completed Form of Acceptance to the Receiving Agent not later than the time and date on which the Offer expires (see question 8 below). If you hold your Celltech Shares in certificated form, you must also deliver your share certificates. If you hold your Celltech Shares in CREST, you must follow the instructions set out in paragraph 18(b) of the letter from Lazard set out in Part II of this document.

If you are a holder of Celltech ADSs, to accept the Offer, you must deliver your Celltech ADRs evidencing your Celltech ADSs, together with a completed Letter of Transmittal, to the Tender Agent not later than the time and date on which the Offer expires (see question 8 below).

If your Celltech ADSs are held in a "street name" in the United States, your nominee can tender them through the applicable book entry transfer system. In addition, in the case of Celltech ADSs, if you cannot get any document or instrument that is required to be delivered by the expiration of the Offer, you may gain some time by following the procedures for guaranteed delivery. See paragraph 18(c) of the letter from Lazard and paragraph 2 of Part C of Appendix I to this document.

5      How does the Offer compare with recent prices of Celltech Shares and Celltech ADSs?

The Offer for Celltech Shares represents a premium of:

•

27.8 per cent. to the Closing Price of 430.5 pence per Celltech Share on 17 May 2004, the last business day prior to the announcement by Celltech of the Offer;



•

26.3 per cent. to the average Closing Price of 435.6 pence per Celltech Share during the three months prior to 17 May 2004; and



•

44.7 per cent. to the average Closing Price of 380.1 pence per Celltech Share during the twelve months prior to 17 May 2004.

See paragraph 3 of Appendix V to this document for the variation in the prices of Celltech Shares and Celltech ADSs.

1

6      Do the Directors of Celltech support the Offer?

Yes. The Celltech Board, which has been so advised by Morgan Stanley and JPMorgan, considers the terms of the Offer to be fair and reasonable. In providing advice to the Celltech Board, Morgan Stanley and JPMorgan have taken into account the commercial assessments of the Celltech Board. Accordingly, the Celltech Board unanimously recommends that Celltech Shareholders accept the Offer, as they and certain members of their families have irrevocably undertaken to do in respect of Celltech Shares beneficially owned and controlled by them. See the letter from the Chairman of Celltech in the next section of this document.

7      Does UCB have the financial resources to make payment?

Yes. The Offer will be financed from funds made available to UCB pursuant to financing agreements. The Offer is not conditional upon any financing arrangements. See paragraph 7 of Appendix V to this document.

8      How long do I have to accept the Offer?

You will have until 3.00 p.m. (London time), 10.00 a.m. (New York City time), on 17 June 2004 to accept the Offer or withdraw your acceptance, unless the Initial Offer Period is extended. In addition, you may accept the Offer but not withdraw your acceptance during the Subsequent Offer Period except in the limited circumstances described in paragraph 3 of Part B of Appendix I. If you are a holder of Celltech ADSs and you cannot deliver everything that is required in order to make a valid tender of Celltech ADSs by that time, you may be able to use a Guaranteed Delivery Procedure, which is described later in this document. See paragraph 18(c) of the letter from Lazard and paragraph 2 of Part C of Appendix I to this document.

9      Until what time can I withdraw my acceptance?

The Initial Offer Period for acceptances and withdrawals is the period from the date of this document until the time and date (not being before 3.00 p.m. (London time), 10.00 a.m. (New York City time)), on 17 June 2004 and not, except with the consent of the Panel, being after 1.00 p.m. (London time), 8.00 a.m. (New York City time) on 18 July 2004) on which date all the Conditions are satisfied, fulfilled or, to the extent permitted, waived or, if earlier, the time and date on which the Offer lapses.

Unless the Offer has lapsed, the Subsequent Offer Period starts as soon as the Initial Offer Period terminates. The Subsequent Offer Period must remain open for at least 14 days but it may be extended beyond that time by UCB until a further specified date or until further notice.

You can withdraw your acceptance during the Initial Offer Period but not during the Subsequent Offer Period except in the limited circumstances described in paragraph 3 of Part B of Appendix I. See paragraph 3 of Part B of Appendix I of this document.

10    Can the Offer be extended and under what circumstances?

If all of the Conditions have not been either satisfied, fulfilled or, to the extent permitted, waived by UCB by 3.00 p.m. London time, 10.00 a.m. (New York City time) on 17 June 2004, UCB may choose, but shall not be obliged, to extend the Initial Offer Period. UCB may also be required to extend the Initial Offer Period under applicable UK and US securities laws if it changes the Offer in any material respect. The Initial Offer Period for acceptances and withdrawals cannot be extended beyond 1.00 p.m. (London time), 8.00 a.m. (New York City time) on 18 July 2004 without the consent of the Panel.

Once all the Conditions have been either satisfied, fulfilled or, to the extent permitted, waived by UCB it will extend the Offer for a Subsequent Offer Period of at least 14 days. See paragraph 1 of Part B of Appendix I to this document.

11    How will I be notified if the Offer is extended?

If UCB extends the Offer, it will make a public announcement of the extension not later than 8.00 a.m. (London time) in the United Kingdom, and 8.00 a.m. (New York City time) in the United States, on the next business day after the date on which the Offer was scheduled to expire. See paragraph 1 of Part B of Appendix I to this document.

UCB will also announce by not later than 8.00 a.m. (London time) in the United Kingdom, and 8.00 a.m. (New York City time) in the United States, on the business day following the end of the

2

Initial Offer Period that there will be a Subsequent Offer Period. The Subsequent Offer Period will remain open for at least 14 days but UCB may extend it beyond that time until a further specified date or until further notice.

12    What are the most significant conditions to the Offer?

Unless it has received valid acceptances (which have not been properly withdrawn) in respect of at least 90 per cent. of the Celltech Shares (including Celltech Shares represented by Celltech ADSs) to which the Offer relates, UCB is not obliged to purchase any Celltech Shares and/or Celltech ADSs. This percentage may be reduced at the discretion of UCB, subject to certain limits. At least five US business days prior to any reduction, UCB will announce that it may do this through a press release and an advertisement in a newspaper with general circulation in the United States.

UCB is not obliged to purchase any Celltech Shares and/or Celltech ADSs unless, among other things, each of the antitrust authorities in Germany and Austria approve the transaction on terms reasonably satisfactory to UCB. In relation to the United Kingdom, the Offer is conditional on the OFT indicating either that they do not have jurisdiction over the transaction or do not intend to refer the transaction to the Competition Commission and in relation to the US, the Offer is conditional on a filing being made and the applicable waiting period under the United States Hart-Scott-Rodino Antitrust Improvements Act of 1976 having expired or been terminated.

See Part A of Appendix I to this document.

13    How do I withdraw my acceptance?

To withdraw an acceptance in relation to the Offer for Celltech Shares or Celltech ADSs, you must deliver a written notice of withdrawal with the required information to the Receiving Agent or the Tender Agent, as applicable, while you still have the right to withdraw the Celltech Shares or Celltech ADSs. See paragraph 3 of Part B of Appendix I to this document.

14    Will the Offer be followed by a compulsory acquisition?

If all of the Conditions to the Offer are either satisfied, fulfilled or, where permitted, waived and UCB has acquired 90 per cent. in nominal value of Celltech Shares (including Celltech Shares represented by Celltech ADSs) within the statutory time period, then UCB will be entitled to and intends to acquire all remaining Celltech Shares (including those represented by Celltech ADSs) pursuant to the Companies Act. Celltech Shareholders and holders of Celltech ADSs subject to the compulsory acquisition would be offered the same consideration as those Celltech Shareholders and holders of Celltech ADSs who accept the Offer. See paragraph 7(c) of Part B of Appendix I to this document.

15    If I decide not to accept, how will the Offer affect my securities?

If UCB is able to, it will acquire all Celltech Shares (including Celltech Shares represented by Celltech ADSs) for which it has not received acceptances pursuant to the compulsory acquisition provisions of the Companies Act. UCB also intends to procure the making of an application by Celltech for the removal of Celltech Shares from the Official List and for the cancellation of trading in Celltech Shares on the London Stock Exchange's market for listed securities. It is anticipated that cancellation of listing and trading will take effect no earlier than 20 Business Days after the Offer becomes or is declared unconditional in all respects. UCB also intends to procure that Celltech applies for de-listing of the Celltech ADSs from the New York Stock Exchange. Such de-listings and cancellation would significantly reduce the liquidity and marketability of any Celltech Securities not tendered in the Offer. UCB may also request that Celltech terminate the existing deposit agreement through which the ADS programme is operated. See paragraph 7(d) of Part B of Appendix I to this document.

16    Can I choose the currency of the cash that I receive?

If you accept the Offer for Celltech Shares, you will receive the price for your shares in pounds sterling.

If you accept the Offer for Celltech ADSs, you will receive the price for your ADSs in US dollars unless you specifically elect to receive it in pounds sterling.

Where you are receiving US dollars, the cash amount payable in pounds sterling to which you would otherwise be entitled pursuant to the terms of the Offer will be converted, without charge, from pounds sterling to US dollars at the exchange rate obtainable on the spot market in London on the date the cash consideration is made available by UCB to the Tender Agent for delivery in respect of

3

your Celltech ADSs. The actual amount of US dollars received will depend upon the exchange rate prevailing on the day on which funds are made available to the Tender Agent by UCB.

See paragraph 20 of the letter from Lazard and paragraph 2(j) of Part C of Appendix I to this document.

17    Will I have to pay any fees or commissions?

If you are the registered owner of your Celltech Shares and/or Celltech ADSs and you accept the Offer, you will not have to pay brokerage fees or similar expenses. If you own your Celltech Shares and/or Celltech ADSs through a broker or other nominee, and your broker accepts the Offer on your behalf, your broker or nominee may charge you a fee for doing so. You should consult your broker or nominee to determine whether any charges will apply.

18    Will I be taxed on the cash that I receive?

For UK tax purposes, a UK resident holder who accepts the Offer and elects for the cash consideration (to the extent that cash consideration, rather than Loan Notes, is received in return for those Celltech Shares) will generally realise an immediate chargeable gain or allowable loss if the Offer becomes unconditional.

For US federal income tax purposes, a US Holder will generally recognise a capital gain or loss on the exchange of Celltech Shares or Celltech ADSs in an amount equal to the difference between the US Holder's tax basis in its Celltech Shares or Celltech ADSs and the offer consideration valued in US dollars. A US Holder may also recognise an exchange gain or loss due to currency fluctuations.

Further information regarding the application of both UK and US tax laws to holders of Celltech Securities who accept the Offer is set out in paragraph 16 of the letter from Lazard set out in Part II of this document.

19    Is there an alternative to cash consideration for my Celltech Securities?

As part of the Offer, Celltech Shareholders who are eligible to do so may elect to receive some or all of their consideration in Loan Notes. For UK tax purposes, the effect of electing for the Loan Note Alternative should be to allow eligible Celltech Shareholders to defer any taxable gain arising on a disposal of their Celltech Shares (to the extent that Loan Notes, rather than cash, are received in return for those Celltech Shares) until such time as the Loan Notes are transferred or redeemed. Such tax treatment is not available for the Loan Note Alternative under US federal income tax laws and the Loan Note Alternative is not available to Celltech Shareholders who are US persons or persons resident in a Restricted Jurisdiction. The Loan Note Alternative is also not available to holders of Celltech ADSs.

Further information relating to the Loan Note Alternative is set out in paragraph 3 of the letter from Lazard and Appendix II to this document.

20    Who can answer questions I might have about the Offer?

If you have any questions about procedures for acceptance of the Offer, you should contact the Helpline on one of the following numbers:

•

From the United Kingdom*: 0870 162 3118



•

From other countries: +44 208 639 2157



•

Toll free in the United States: (800) 261 1054

*

Call charged at national rate

4

PART I

LETTER FROM THE CHAIRMAN OF CELLTECH

        19 May 2004

To:    Celltech Shareholders and holders of Celltech ADSs and, for information only, to participants in the Celltech Share Plans

Dear Shareholder or ADS holder,

RECOMMENDED CASH OFFER BY UCB FOR CELLTECH

1      Introduction

        The boards of UCB and Celltech announced on 18 May 2004 that they have agreed the terms of a recommended cash offer for the entire issued and to be issued share capital of Celltech.

        I am now writing to you to set out the terms of the Offer, to explain the background to and the reasons for our recommendation of the Offer and to seek your acceptance of the Offer.

2      Summary Terms of the Offer

        The Offer (on the terms and subject to the conditions set out in this document and in the Acceptance Forms) is contained in the letter from Lazard, UCB's financial adviser, set out in Part II of this document.

        Under the terms of the Offer, holders of Celltech Securities will receive:

        This represents:

        The Offer values the entire issued and to be issued share capital of Celltech at approximately £1,530 million.

        The Offer represents a premium of:

•

27.8 per cent. to the Closing Price of 430.5 pence per Celltech Share on 17 May 2004, the last business day prior to the announcement by Celltech of the Offer;

•

26.3 per cent. to the average Closing Price of 435.6 pence per Celltech Share during the three months prior to 17 May 2004; and

•

44.7 per cent. to the average Closing Price of 380.1 pence per Celltech Share during the twelve months prior to 17 May 2004.

        The Celltech Shares (including those represented by Celltech ADSs) will be acquired pursuant to the Offer fully paid and free from all liens, charges, equitable interests, encumbrances, rights of pre-emption

Celltech Group plc    208 Bath Road    Slough    Berkshire SL1 3WE    United Kingdom
Tel: +44(0) 1753 534655    Fax: +44(0) 1753 536632    www.celltechgroup.com
Registered Office as above.    Registered in England No. 2159282

5

and any other rights and interests of any nature whatsoever and together with all rights now and hereafter attaching thereto, including voting rights and the right to receive and retain in full all dividends and other distributions (if any) declared, made or paid on or after the date of the Announcement.

3      Loan Note Alternative

        As an alternative to some or all of the cash consideration of 550 pence per Celltech Share which they would otherwise receive under the Offer, Celltech Shareholders (other than certain overseas shareholders) who validly accept the Offer will be able to elect to receive Loan Notes. The Loan Note Alternative will be made available on the following basis:

for every whole £1 in cash consideration £1 nominal value of Loan Notes

        A summary of the terms of the Loan Note Alternative is set out in the letter from Lazard in Part II of this document and further details of the Loan Notes are set out in Appendix II to this document.

4      Information on Celltech

        Celltech is one of the largest European-based biopharmaceutical companies, possessing significant discovery and development capabilities, a broad product pipeline, and an international pharmaceutical business, with operations in the United States and Europe. It derives revenues from the licensing of its technologies and products and the sale of pharmaceutical products through its pharmaceutical business.

        The discovery and development activities are focused on treatments for auto-immune and inflammatory disorders and oncology. Its pipeline includes candidates comprising new chemical entities and antibody-based therapeutics, in pre-clinical or clinical development and marketing licence registration. Its technology base includes a leading position in antibody engineering and extensive medicinal chemistry capabilities. Celltech has a range of discovery, development and commercialisation collaborations with leading pharmaceutical and biotechnology companies including: Abgenix, Amgen, AstraZeneca, Biogen Idec, Johnson & Johnson, Merck, NeoGenesis, Seattle Genetics and Wyeth.

        Celltech Shares are traded on the London Stock Exchange and Celltech ADSs are listed on the New York Stock Exchange. Cazenove & Co. Limited acts as broker to Celltech.

5      Celltech current trading

        Celltech announced its preliminary results for the year ended 31 December 2003 on 16 March 2004 and posted the annual report and accounts for such period to Celltech Shareholders during the week commencing 19 April 2004. For the year ended 31 December 2003, in accordance with generally accepted accounting principles in the United Kingdom, Celltech reported turnover of £353.3 million (2002: £329.6 million) with operating profit pre exceptional items and goodwill of £49.5 million (2002: £49.0 million) and net assets of £505.9 million (2002: £564.4 million). The Celltech Board considers that Celltech's business continues to perform in line with the Celltech Board's expectations.

6      Background to and reasons for recommending the Offer

        On 1 December 2003, Celltech announced that it had regained full control of CDP870 following termination of its previous collaboration agreement with Pfizer. Celltech subsequently was contacted by a number of potential partners with a view to entering into a new collaboration agreement. As announced on 31 March 2004, Celltech received expressions of interest from a range of leading pharmaceutical and biotechnology companies and over recent weeks entered detailed late stage discussions with a number of parties.

        As part of these discussions, UCB proposed terms for a collaboration agreement which the Celltech Board considered to be the best route for the successful development and commercialisation of CDP870 given the terms proposed, the strength of UCB's specialist sales network and the relevant expertise of UCB's senior management. Celltech's management believe that, based on like for like commercial assumptions, the CDP870 agreement with UCB is of broadly equivalent value to the prior agreement with Pfizer. This agreement with UCB, which is not conditional on completion of the Offer, was announced on 18 May 2004.

        During the course of the CDP870 discussions, UCB also proposed combining the Celltech and UCB businesses in their entirety by way of an offer for Celltech by UCB. Having considered UCB's Offer, the Celltech Board has concluded that the Offer, which was announced on 18 May 2004 and full details of

6

which are set out in this document, should unanimously be recommended to shareholders. For further information regarding the background to the Offer, see paragraph 9(h) of Appendix V to this document.

7      Management and Employees

        UCB has assured the Celltech Board that, following the Offer becoming or being declared unconditional in all respects, the existing employment rights, including pension rights, of all employees of the Celltech Group will be fully safeguarded.

8      Celltech Share Plans

        The Offer will (subject to compliance with any applicable local laws) extend to any Celltech Shares issued fully paid (or credited as fully paid) or unconditionally allotted or issued while the Offer remains open for acceptance (or such earlier date as UCB may, subject to the City Code and the Exchange Act, determine, not being earlier than the date on which the Offer becomes unconditional as to acceptances), as a result of the exercise of options granted under the Celltech Share Plans. As soon as practicable after the Offer becomes or is declared unconditional in all respects, appropriate proposals will (taking into account any local laws) be made to the holders of options under the Celltech Share Plans.

9      Irrevocable undertakings

        UCB has received irrevocable undertakings to accept (or procure the acceptance of) the Offer from Celltech Directors and certain members of their immediate families in respect of, in aggregate, 493,029 Celltech Shares, representing all of the Celltech Shares beneficially owned and controlled by them and representing approximately 0.18 per cent. of the existing issued share capital of Celltech. Such undertakings cease to be binding only if the Offer lapses or is withdrawn.

10    Inducement Fee

        Celltech and UCB have entered into an agreement under which Celltech has agreed to pay UCB a sum of £15.25 million in certain circumstances. Further details of the inducement fee arrangement are set out in paragraph 15 in the letter from Lazard set out in Part II of this document and in paragraph 9(a) of Appendix V of this document.

11    Taxation

        Your attention is drawn to paragraph 16 of the letter from Lazard set out in Part II of this document. If you are in any doubt as to your tax position or are subject to taxation in any jurisdiction other than the United Kingdom or the United States, you should consult an appropriate professional adviser immediately.

12    Action to be taken to accept the Offer

        Your attention is drawn to the letter from Lazard in Part II of this document, the Appendices to this document and the accompanying Form of Acceptance or (if you are a holder of Celltech ADSs) Letter of Transmittal. The procedure for acceptance of the Offer in relation to Celltech Shares is set out in paragraph 18 of the letter from Lazard in Part II of this document and in the Form of Acceptance or (if you are a holder of Celltech ADSs) the Letter of Transmittal.

        A separate step-by-step guide on how to fill in your Form of Acceptance has been enclosed with this document.

        If you are a holder of Celltech Shares, in order to accept the Offer, you should complete and return the accompanying Form of Acceptance, whether or not your Celltech Shares are in CREST, in accordance with the instructions thereon as soon as possible and, in any event, so as to be received by post or by hand (during normal business hours only) by Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH, by no later than 3.00 p.m. (London time) on 17 June 2004.

        If you require further assistance on how to complete the Form of Acceptance, please call the Helpline. Please note that the Helpline will be unable to advise you on whether or not to accept the Offer or whether you should elect for the Loan Note Alternative.

7

        Your decision as to whether to elect to receive cash or Loan Notes will depend on your individual circumstances, including your tax position. Paragraph 16 of the letter from Lazard set out in Part II of this document sets out certain implications of acceptance of the Offer in relation to United Kingdom and United States taxation. If you are in any doubt about the actions you should take, you should immediately consult your stockbroker, bank manager, solicitor, accountant or other independent financial adviser, duly authorised under the Financial Services and Markets Act 2000, if you are resident in the United Kingdom or, if not, another appropriately authorised independent financial adviser.

        If you are a holder of Celltech ADSs, in order to accept the Offer you should complete the Letter of Transmittal in accordance with paragraph 18(c) of the letter from Lazard and paragraph 2 of Part C of Appendix I to this document. If you require further assistance on how to complete the Letter of Transmittal, please call the Helpline.

13    Recommendation

        The Celltech Board, which has been so advised by Morgan Stanley and JPMorgan, considers the terms of the Offer to be fair and reasonable. In providing advice to the Celltech Board, Morgan Stanley and JPMorgan have taken into account the commercial assessments of the Celltech Board.

        Accordingly, the Celltech Board unanimously recommends that Celltech Shareholders accept the Offer, as Celltech Directors and certain members of their immediate families have irrevocably undertaken to do in respect of all of the Celltech Shares beneficially owned and controlled by them representing, in aggregate, 493,029 Celltech Shares and representing approximately 0.18 per cent. of the issued share capital of Celltech.

Yours faithfully

Dr Peter Fellner
Chairman

8

PART II

LETTER FROM LAZARD

LAZARD & CO., LIMITED 50 Stratton Street, London W1J 8LL Authorised and regulated by the Financial Services Authority Member of the London Stock Exchange Registered in England no. 162175

        19 May 2004

To:    Celltech Shareholders and to the holders of Celltech ADSs and, for information only, to participants in the Celltech Share Plans

Dear Shareholder or ADS holder,

RECOMMENDED CASH OFFER BY UCB FOR CELLTECH

1      Introduction

        On 18 May 2004, the boards of UCB and Celltech announced that they had reached agreement on the terms of a recommended cash offer for the entire issued and to be issued ordinary share capital of Celltech. The Offer is being made (outside the United States) by Lazard on behalf of UCB and (in the United States) by UCB itself. The offer and this document are subject to the applicable requirements of both the City Code and the Exchange Act, subject to customary exemptions granted by the SEC in relation to the Offer.

        Your attention is drawn to the letter of recommendation from the Chairman of Celltech in Part I of this document, which sets out the reasons why the Celltech Board, which has been so advised by Morgan Stanley and JPMorgan, considers the terms of the Offer to be fair and reasonable and unanimously recommends that all Celltech Shareholders accept the Offer, as Celltech Directors and certain members of their immediate families have irrevocably undertaken to do (or procure to be done) in respect of all of the Celltech Shares beneficially owned and controlled by them amounting to, in aggregate, 493,029 Celltech Shares, representing approximately 0.18 per cent. of the existing issued share capital of Celltech.

        In providing advice to the Celltech Board, Morgan Stanley and JPMorgan have taken into account the commercial assessments of the Celltech Board.

2      Summary of the Offer

        Lazard, on behalf of UCB (outside the United States), and UCB itself (inside the United States) offer to acquire (on the terms and subject to the conditions set out in this document and in the Acceptance Forms) the entire issued and to be issued share capital of Celltech for cash. The Offer is being made on the following basis:

        The Offer values the entire issued and to be issued share capital of Celltech at approximately £1,530 million.

        The Offer represents a premium of:

•

27.8 per cent. to the Closing Price of 430.5 pence per Celltech Share on 17 May 2004, the last business day prior to the announcement by Celltech of the Offer;

•

26.3 per cent. to the average Closing Price of 435.6 pence per Celltech Share during the three months prior to 17 May 2004; and

•

44.7 per cent. to the average Closing Price of 380.1 pence per Celltech Share during the twelve months prior to 17 May 2004.

        Celltech Shares (including those represented by Celltech ADSs) will be acquired pursuant to the Offer fully paid and free from all liens, charges, equitable interests, encumbrances, rights of pre-emption

9

and other third party rights or interests of any nature whatsoever and together with all rights attaching now or hereafter attaching thereto including, without limitation, voting rights and the right to receive and retain all dividends and other distributions (if any), declared, made or paid on or after the date of the Announcement.

        The Offer (including the Loan Note Alternative) will be subject to the conditions and further terms set out or referred to in Appendix I to this document and in the Acceptance Forms.

        If you are a Celltech Shareholder, to accept the Offer you should return the Form of Acceptance whether or not your Celltech Shares are in CREST, together with all other required documents (such as your share certificate(s)), as soon as possible and, in any event, so as to be received by Capita IRG Plc, Corporate Actions, PO Box 166, The Registry, 34 Beckenham Road, Beckenham, Kent BR3 4TH by no later than 3.00 p.m. (London time) on 17 June 2004.

        The procedure for acceptance of the Offer if you are a Celltech Shareholder is set out in paragraph 18 below and in paragraph 1 of Part C of Appendix I to this document, and in the accompanying Form of Acceptance.

        In addition, a separate step-by-step guide to completing the Form of Acceptance has been sent to you with this document.

        If you are a holder of Celltech ADSs, you should follow the instructions set out in paragraph 18(c) below and in paragraph 2 of Part C of Appendix I to this document.

3      Loan Note Alternative

        Celltech Shareholders (other than certain Overseas Shareholders) who validly accept the Offer will be able to elect to receive Loan Notes instead of some or all of the cash to which they would otherwise become entitled under the terms of the Offer. The Loan Note Alternative is being made available on the following basis:

for every whole £1 in cash consideration £1 nominal value of Loan Notes

        The Loan Notes, which will be governed by English law, will be unsecured and will be issued credited as fully paid in amounts and integral multiples of £1 nominal value. All fractional entitlements to the Loan Notes will be disregarded. No application will be made for the Loan Notes to be listed or dealt on any stock exchange.

        The Loan Notes will bear interest at 0.75 per cent. below six month sterling LIBOR (as described in paragraph 2 of Appendix II). Interest will be payable by six-monthly instalments in arrears (less any tax required to be withheld) on 31 March and 30 September in each year. The first payment of interest will be made on 31 March 2005 (the "First Payment Date"). On the First Payment Date, interest will be paid in respect of the period from (and including) the first date of issue of any of the Loan Notes to (but excluding) the First Payment Date. The Loan Notes will be redeemable in whole or in part for cash at the option of Noteholders on the First Payment Date and on subsequent interest payment dates. No Loan Notes may be redeemed before the First Payment Date. In certain circumstances (set out in the Loan Note Instrument), UCB will have the right to redeem all of the Loan Notes. If not previously redeemed, all outstanding Loan Notes will be redeemed on 30 September 2010.

        No Loan Notes will be issued unless, on or before the date on which the Offer becomes or is declared unconditional in all respects, valid elections have been received in respect of at least £5 million in nominal value of Loan Notes. If insufficient elections are received, Celltech Shareholders electing for the Loan Note Alternative will instead receive cash in accordance with the terms of the Offer.

        Subject as aforesaid, the Loan Note Alternative will remain open for acceptance for so long as the Offer remains open for acceptance. The Loan Note Alternative will be conditional upon the Offer becoming or being declared unconditional in all respects.

        Lazard, financial adviser to UCB, has advised that based on market conditions on 18 May 2004 (the last practicable date prior to the posting of this document), in its opinion, if the Loan Notes had been in issue on that date, the value of each £1 nominal of Loan Notes would have been approximately 99 pence.

        Celltech Shareholders and holders of Celltech ADSs who are not resident in the United Kingdom should refer to paragraph 17 below and paragraph 6 of Part B of Appendix I.

10

4      Irrevocable undertakings

        UCB has received irrevocable undertakings to accept (or procure the acceptance of) the Offer from Celltech Directors and certain members of their immediate families in respect of all of the Celltech Shares beneficially owned and controlled by them amounting to, in aggregate, 493,029 Celltech Shares, representing approximately 0.18 per cent. of the existing issued share capital of Celltech.

5      Background to and reasons for the Offer

        The combination of UCB and Celltech will create a European-based company which will be one of the largest biopharmaceutical companies in the world. In a number of important areas the combined group will benefit from a high degree of complementarity:

•

complementary product offerings: the combined group will have strong positions in specialty therapeutic areas such as Central Nervous System (Epilepsy, Multiple Sclerosis, Parkinson's Disease, etc.), Inflammation (Respiratory, Rheumatology, Gastro-Enterology) and Oncology;

•

strengthened research and development: the combined group will benefit from Celltech's innovative expertise in biotechnological R&D, especially monoclonal antibodies, and UCB's expertise in pharmaceutical chemistry. This will result in a significantly strengthened combination of small and large molecule discovery and development expertise allowing the combined group to increase the flow of drug development candidates;

•

stronger and broader commercial operations (US, Europe and Asia): the combined group will be better positioned, through its enhanced global presence, to commercialise and launch new products, particularly in the specialist areas in which the combination will focus and where UCB has already demonstrated its ability to launch and propel products successfully to market leadership (e.g. Keppra); and

•

the blending of skills and culture, through the successful combination of competencies at all levels of the combined entity, will accelerate innovation and profitable growth.

        The R&D operations of the combined group will have its headquarters in Slough, UK.

        Dr Göran Ando will be deputy chief executive officer of the combined group, Mr Peter Allen will be in charge of integration and Dr Melanie Lee will be head of the combined group's R&D operations.

        On 18 May 2004, UCB and Celltech entered into a co-exclusive world-wide collaboration agreement for the research, development and commercialisation of CDP870, Celltech's anti-TNF-alpha PEGylated antibody fragment, for all indications outside of Crohn's disease in North America and major European markets. This collaboration agreement is not conditional upon the success of the Offer for Celltech by UCB.

6      Financial Effects of the Transaction

        The cash consideration due under the Offer will be financed from the facilities described in paragraph 7 of Appendix V. It is expected that the transaction will be earnings accretive post synergies and before goodwill and other intangibles amortisation, after the second full year.

7      Information on the Celltech Group and Celltech current trading

        Celltech is one of the largest European-based biopharmaceutical companies, possessing significant discovery and development capabilities, a broad product pipeline, and an international pharmaceutical business, with operations in the United States and Europe. It derives revenues from the licensing of its technologies and products and the sale of pharmaceutical products through its pharmaceutical business.

        The discovery and development activities are focused on treatments for auto-immune and inflammatory disorders and oncology. Its pipeline includes candidates comprising new chemical entities and antibody-based therapeutics, in pre-clinical or clinical development and marketing licence registration. Its technology base includes a leading position in antibody engineering and extensive medicinal chemistry capabilities. Celltech has a range of discovery, development and commercialisation collaborations with leading pharmaceutical and biotechnology companies including: Abgenix, Amgen, AstraZeneca, Biogen Idec, Johnson & Johnson, Merck, NeoGenesis, Seattle Genetics and Wyeth.

11

        Celltech Shares are traded on the London Stock Exchange and Celltech ADSs are listed on the New York Stock Exchange. Cazenove & Co. Limited acts as broker to Celltech.

        Celltech announced its preliminary results for the year ended 31 December 2003 on 16 March 2004 and posted the annual report and accounts for such period to Celltech Shareholders during the week commencing 19 April 2004. For the year ended 31 December 2003, in accordance with generally accepted accounting principles in the United Kingdom, Celltech reported turnover of £353.3 million (2002: £329.6 million) with operating profit pre exceptional items and goodwill of £49.5 million (2002: £49.0 million) and net assets of £505.9 million (2002: £564.4 million). The Celltech Board considers that Celltech's business will continue to perform in line with the Celltech Board's expectations.

        Financial information on the Celltech Group is contained in Appendix III to this document.

8      Information on the UCB Group and UCB current trading

        UCB is a world-class pharmaceutical and specialty chemical company. UCB is headquartered in Brussels (Belgium) and employs about 11,500 people, of whom more than 6,600 are in the pharmaceutical sector.

        UCB operates in two industrial sectors: Pharma and Surface Specialties. It currently comprises the parent company, UCB S.A., together with about 120 subsidiaries and associated companies in Europe, the Americas and Asia. UCB also operates on a worldwide basis through its agents, distributors and licensees.

        The Pharma Sector researches, produces and markets prescription medical products, particularly in the fields of Allergy/Asthma and Neurology. Over three quarters of the UCB Group's expenditure on research and development is in the Pharma Sector, where it accounts for about 15 per cent. of turnover. UCB is also present in biotechnology, through UCB-Bioproducts, making available peptides by extraction or synthesis to the scientific community.

        Surface Specialties is focused on the manufacture of technically innovative products and solutions for surface applications. There are two business units: Coating Resins & Additives and Films & Adhesives.

        UCB's shares are traded on Euronext. Based on the Closing Price of Euro 35.10 per UCB Share on 17 May 2004 (the last business day prior to the date of the Announcement), UCB has a market capitalisation of approximately Euro 5.1 billion.

        For the year ended 31 December 2003, in accordance with generally accepted accounting principles in Belgium, UCB reported turnover of Euro 2,966 million (2002: Euro 2,514 million) with group profit before interest, tax and exceptional items of Euro 487 million (2002: Euro 503 million) and net assets of Euro 1,784 million (2002: Euro 1,565 million).

        Further to UCB's press release of 4 February 2004, UCB still remains very confident about the favourable prospects for its results (at constant exchange rates), which will be influenced, on one hand by the growth in pharmaceutical specialties (Neurology and Allergy) and, on the other hand, by an increased contribution from synergies generated in the Surface Specialties sector.

        Financial information on UCB is contained in Appendix IV to this document.

9      Management and employees

        The Board of UCB has confirmed that, following the Offer being declared unconditional in all respects, the existing employment rights, including pension rights, of all employees of the Celltech Group will be fully safeguarded.

10    The Celltech Directors and the effect of the Offer on their interests

        Details of the interests of the Celltech Directors in Celltech Shares are set out in paragraph 4 of Appendix V of this document. The effect of the Offer on such interests does not differ from its effect on the interests of any other holder of Celltech Shares or participant in the Celltech Share Plans.

11    Disclosure of Interests in Celltech

        Save for the 493,029 Celltech Shares in respect of which UCB has received irrevocable undertakings to accept the Offer, neither UCB nor any of the directors of UCB nor any other UCB subsidiary, nor, so far as UCB is aware, any person acting in concert with UCB for the purposes of the Offer, owns, controls

12

or holds any Celltech Shares or any securities convertible or exchangeable into, or rights to subscribe for, purchase or holds any options to purchase any Celltech Shares or has entered into any derivative referenced to Celltech Shares which remains outstanding.

12    Celltech Share Plans

        Appropriate proposals in relation to the arrangements for participants in the Celltech Share Plans will be sent to optionholders as soon as practicable after the Offer has become or is declared unconditional in all respects.

        All existing options under the Celltech Share Plans, which are not already exercisable, will generally become exercisable in accordance with the rules of the Celltech Share Plans for a period ranging from one to six months (depending on the rules of the particular plan) following the Offer becoming, or being declared, unconditional in all respects. This period may be reduced if UCB commences the compulsory acquisition procedure.

        If an optionholder does not accept the Offer in respect of Celltech Shares acquired under any Celltech Share Plan, it is UCB's intention to acquire compulsorily such shares as described in paragraph 14 below. Any options which are not exercised will lapse no later than six months following the Offer becoming, or being declared, wholly unconditional (depending on the rules of the particular plan).

13    Collaboration Agreement

        On 18 May 2004, Celltech announced that it had entered into an agreement with UCB for the world-wide development and marketing of CDP870, Celltech's anti-TNF-alpha PEGylated antibody fragment.

        Under the terms of this agreement, Celltech granted UCB co-exclusive worldwide rights to develop and commercialise CDP870. The licence is exclusive for rheumatoid arthritis and other indications, excluding Crohn's disease. UCB will be responsible for the conduct of future clinical studies and all commercialisation activities with CDP870 other than in Crohn's disease, and will pay Celltech a significant royalty on sales in these indications. UCB will also make progress-related payments to Celltech dependent upon attaining certain project related milestones. Celltech has retained manufacturing rights and will supply all CDP870 material for commercialisation, and will discharge all royalties due to third parties. Celltech has retained exclusive rights for the development and commercialisation of CDP870 in Crohn's disease in North America, major European markets, Australia and New Zealand, with UCB having development and commercialisation rights in other territories.

        This agreement is not conditional upon the success of the Offer for Celltech by UCB.

14    Compulsory acquisition, delisting and re-registration

        If UCB receives acceptances under the Offer in respect of, and/or otherwise acquires, 90 per cent. or more of the Celltech Shares to which the Offer relates and the Offer becomes or is declared unconditional in all respects, UCB intends to exercise its rights pursuant to the provisions of sections 428 to 430F (inclusive) of the Companies Act to acquire compulsorily the remaining Celltech Shares to which the Offer relates.

        After the Offer becomes or is declared unconditional in all respects, UCB intends to procure the making of an application by Celltech to the UKLA for the cancellation of the listing of the Celltech Shares on the Official List and to the London Stock Exchange for the cancellation of the admission to trading of the Celltech Shares on its market for listed securities. It is anticipated that cancellation of listing and trading will take effect no earlier than 20 Business Days after the Offer becomes or is declared unconditional in all respects. UCB also intends to procure that Celltech applies for delisting of the Celltech ADSs from the New York Stock Exchange. Such delistings would significantly reduce the liquidity and marketability of any Celltech Securities not tendered into the Offer. UCB may also request that Celltech terminate the existing deposit agreement through which the ADS programme is operated.

        It is also proposed that, following the Offer becoming or being declared unconditional in all respects and after the Celltech Shares are delisted, Celltech will be re-registered as a private company under the relevant provisions of the Companies Act.

13

15    Inducement fee

        Celltech and UCB have entered into an agreement under which Celltech has agreed to pay to UCB a sum of £15.25 million (being approximately one per cent. of the value of the Offer) in the event that (a) the Celltech Directors withdraw their unanimous recommendation of the Offer or recommend an alternative transaction, resulting in any person other than UCB (or any person acting in concert with UCB as defined in the City Code) acquiring control of Celltech (as defined in the City Code) or a substantial part of the business of Celltech; or (b) before the Offer lapses or is withdrawn without becoming or being declared wholly unconditional, any person (other than UCB or a person acting in concert (as defined in the City Code) with UCB) announces an intention to make a competing offer however effected, to acquire the entire issued share capital of Celltech (other than Celltech Shares owned by such third party or persons acting in concert with it) and, at any time, the competing offer becomes or is declared wholly unconditional.

16    Taxation

(A)  UK Taxation

        The following paragraphs, which are intended as a general guide only and are based on current UK legislation and Inland Revenue practice, summarise certain limited aspects of the UK taxation consequences of acceptance of the Offer. They relate only to the position of Celltech Shareholders who (unless the position of non-UK resident Celltech Shareholders is expressly referred to) are resident in or individuals ordinarily resident in the United Kingdom for tax purposes and who hold their Celltech Shares beneficially as an investment.

(i)    Taxation of Chargeable Gains

        Liability to United Kingdom taxation of chargeable gains in respect of the disposal of Celltech Shares pursuant to the Offer will depend on each Celltech Shareholder's individual circumstances and on whether a Celltech Shareholder accepts the consideration in cash or elects for the Loan Note Alternative.

(a)   Cash

To the extent a Celltech Shareholder receives cash under the Offer, this will constitute a disposal or part disposal of his Celltech Shares for the purposes of United Kingdom taxation of chargeable gains. Such a disposal may give rise to a liability for United Kingdom taxation of chargeable gains depending on the Celltech Shareholder's individual circumstances (including the availability of exemptions or allowable losses).

An alternative treatment may be possible where a Celltech Shareholder opts for a mixture of cash and Loan Notes. If the amount of cash received by the Celltech Shareholder is "small", as compared with the value of his Celltech Shares, the receipt of the cash will not, unless the Celltech Shareholder elects otherwise, trigger a disposal at that time. A disposal will be triggered only when his Loan Notes are disposed of and the amount of the cash received will be deducted from his chargeable gains acquisition cost in the Loan Notes. Current Inland Revenue practice is to regard a sum as "small" for these purposes if either (i) it is five per cent. or less of the value of the Celltech Shares held by the particular Celltech Shareholder; or (ii) it is £3,000 or less, regardless of whether it satisfies the five per cent. test. The advisability of adopting this alternative treatment will depend upon a Celltech Shareholder's individual circumstances, in particular the availability to a Celltech Shareholder of any reliefs or exemptions from UK taxation on chargeable gains in the tax year in which the cash is received.

(b)   Loan Notes

To the extent a Celltech Shareholder who (either alone or together with persons connected with him) does not hold more than five per cent. of, or of any class of, the shares in, or debentures of Celltech, receives Loan Notes under the Offer, he should be treated as not having made a disposal of his Celltech Shares for the purposes of UK taxation of chargeable gains.

For an individual or other Celltech Shareholder who is not within the charge to UK corporation tax, the Loan Notes should not constitute qualifying corporate bonds for the purposes of UK taxation of chargeable gains. Accordingly, for such a Celltech Shareholder any gain or loss which would otherwise

14

have arisen on a disposal of his Celltech Shares should be "rolled-over" into the Loan Notes so that the Loan Notes will be treated as the same asset as the Celltech Shares, acquired at the same time as the Celltech Shares and for the same acquisition cost. A disposal (including redemption or repayment) of Loan Notes by such a holder may give rise to a liability to UK taxation of chargeable gains. Any chargeable gain or allowable loss on disposal of the Loan Notes should be calculated taking into account the allowable original cost to the holder of acquiring the relevant Celltech Shares. Indexation allowance on that cost should be available (when calculating a chargeable gain but not an allowable loss) in respect of any period of ownership of the Celltech Shares up to April 1998. Thereafter some taper relief may be available which will reduce the amount of the chargeable gain realised on the disposal.

For a Celltech Shareholder within the charge to corporation tax, the Loan Notes will be qualifying corporate bonds for the purposes of UK taxation of chargeable gains. For such a holder, any gain or loss which would otherwise have arisen on a disposal of its Celltech Shares for a consideration equal to market value at the time of the exchange of the Celltech Shares for Loan Notes will be "held over" and deemed to arise on a subsequent disposal (including redemption or repayment) of the Loan Notes. No indexation allowance will be available for the period of ownership of the Loan Notes and, except to the extent any gain or loss which would have otherwise arisen on the disposal of its Celltech Shares was "held over" and crystallises on a subsequent disposal of the Loan Notes, no chargeable gain or allowable loss will arise on such a disposal. There may, however, be a charge to tax as income (see (b) below).

In certain circumstances, the above rules regarding the "roll-over" or "hold over" of any gain or loss will not apply to a Celltech Shareholder who (either alone or together with persons connected with him), holds more than five per cent. of, or of any class of, the shares in, or debentures of Celltech. Such persons are advised that an application for clearance has been made to the Inland Revenue under Section 138 of the Taxation of Chargeable Gains Act 1992 in respect of the Offer and, provided that such clearance is given, the benefit of the above rules regarding the "roll-over" or "hold over" of any gain or loss, will be available to such Celltech Shareholders. It is not a condition of the Offer that such clearance is obtained.

(ii)   Taxation of Income

        UK resident holders of Loan Notes will generally be liable to UK income tax or corporation tax on the amount of any interest received on their Loan Notes.

        Payments of interest on the Loan Notes may be made without withholding on account of UK income tax but will be made subject to withholding on account of Belgian income tax (currently 15 per cent.), except where holders certify to UCB that they were the legal owner or usufructuary (i.e. the person entitled to the right of enjoyment) of their Loan Notes during the entire interest period, are not resident in Belgium and do not hold the Loan Notes for the purposes of a trade or profession which they carry on in Belgium, and provided that certain other conditions are satisfied (see (C) Belgian Withholding Tax below). Holders of Loan Notes will receive a tax certificate with their Loan Notes to enable them to make this certification. In order to continue to receive payments of interest without withholding on account of Belgian income tax, holders of Loan Notes will be required to make such a certification for each interest payment for so long as they receive interest payments under the Loan Notes.

        UK resident holders of Loan Notes who are unable to, or who do not, provide the form of certificate referred to above will generally receive payments of interest subject to withholding on account of Belgian income tax.

        Where interest is paid subject to withholding on account of Belgian income tax, holders of Loan Notes who were unable to provide the form of certificate referred to above, may be able to claim a credit against their UK tax liability (if any) in respect of the relevant interest payment.

        In the case of individual or other non-corporate holders of Loan Notes, a charge to tax on income under the "accrued income scheme" may arise on a transfer of Loan Notes in respect of which interest has accrued since the preceding interest payment date.

        For a holder of Loan Notes within the charge to UK corporation tax, profits, gains and losses and fluctuations in the value of the Loan Notes (whether attributable to currency fluctuations or otherwise) will be taxed or relieved as income, broadly in accordance with the holder's authorised accounting method.

15

(iii) Celltech Share Plans

        Different tax treatment may apply to Celltech Shareholders who acquire their Celltech Shares by exercising options under the Celltech Share Plans, including a possible charge to income tax when such an option is exercised.

(iv)  Stamp Duty

        No stamp duty or stamp duty reserve tax should be payable by Celltech Shareholders as a result of accepting the Offer or on the issue of the Loan Notes.

        Under current Inland Revenue practice, no stamp duty or stamp duty reserve tax will be payable on the transfer or sale of (or agreement to transfer) Loan Notes.

        The above summary is intended only as a general guide to the taxation position under UK tax legislation and does not constitute tax or legal advice. Any person who is in doubt as to his taxation position or who requires more detailed information should consult his own professional tax adviser.

(B)  US Taxation

        The following summary describes certain material US federal income tax consequences that may be relevant to US Holders of Celltech Securities that are considering the Offer.

(i)

This summary provides general information only and is directed solely at US Holders who hold their Celltech Securities as capital assets and whose functional currency is the US dollar. This summary does not discuss all the tax consequences that may be relevant to US Holders in light of their particular investment circumstances, such as investors subject to special tax rules, including: persons who hold Celltech Securities through partnerships or other pass-through entities, banks or other financial institutions, thrifts, real estate investment trusts, regulated investment companies, insurance companies, dealers or traders in securities or currencies and tax-exempt investors, US Holders who received their Celltech Securities in return for services rendered or in connection with their employment, persons that own (directly or indirectly) 10 per cent. or more of Celltech voting stock or persons that hold their Celltech Securities as part of a hedge, straddle or other integrated transaction. This summary does not include descriptions of any alternative minimum tax consequences or the tax laws of any state or local government or of any foreign government that may be applicable to US Holders. Moreover, this summary does not discuss special tax provisions which may apply to individuals who relinquished their US citizenship or residence. This summary is based on the Internal Revenue Code of 1986, the Treasury regulations promulgated thereunder and administrative and judicial interpretations thereof, all as of the date hereof, and all of which are subject to change, possibly with retroactive effect, or different interpretations. No ruling has been requested from the Internal Revenue Service in connection with the Offer and no assurance can be given that the treatment described herein will be accepted by the Internal Revenue Service or, if challenged, by a US court.

The US federal income tax rules are complex. The discussion set forth below is included for general information only and may not be applicable depending upon a holder's particular situation. US Holders should consult their tax advisers with respect to the current and possibly future federal, state, local and foreign tax consequences to them of accepting the Offer.

(ii)

US Holders who sell their Celltech Securities pursuant to the Offer will generally recognise a taxable gain or loss equal to the difference between the US dollar value of their amount realised and their tax basis in their Celltech Securities. Such gain or loss generally will be US-source capital gain or loss and will be long-term if such US Holders held their Celltech Securities for more than one year. US Holders who receive US dollars in exchange for their Celltech ADSs will have an amount realised equal to the payments they receive. Although the law regarding US Holders who receive foreign currency in exchange for securities in a tender offer is unclear, cash basis taxpayers, or accrual basis taxpayers that make an election, who receive pounds sterling in exchange for their Celltech Securities should have an amount realised equal to the dollar value of the pounds sterling they receive, calculated using the spot rate on the settlement date. Accrual basis taxpayers that do not make an election who receive pounds sterling in exchange for their Celltech Securities should have an amount realised equal to the dollar value of the pounds sterling received, calculated using the spot rate on the date such US Holders' tenders are irrevocably accepted. US Holders who receive pounds sterling may recognise foreign currency exchange gain or loss due to fluctuations in the exchange rate upon a

16

subsequent conversion into US dollars of the pounds sterling received. Any foreign currency exchange gain or loss will be ordinary income or loss and will be US-source gain or loss.

The tax consequences for a US Holder could differ adversely from those described above if Celltech was a passive foreign investment company (a "PFIC") at any time during which the US Holder held Celltech Securities. Celltech's possible status as a PFIC must be determined annually. UCB does not have sufficient information to determine with certainty whether Celltech is or has ever been a PFIC.

If Celltech was a PFIC in any year during which a US Holder held Celltech Securities, the US Holder would be required (i) to pay tax on any gain from the sale of Celltech Securities at ordinary income (rather than capital gains) rates and (ii) to pay a special US addition to tax on gains from the sale of the Securities. Celltech will have been treated as a PFIC in any taxable year in which, after taking into account the income and assets of the corporation and certain subsidiaries pursuant to applicable "lookthrough rules," either (i) at least 75 per cent. of its gross income was "passive income" or (ii) at least 50 per cent. of the average value of its assets was attributable to assets which produce passive income or are held for the production of passive income. US Holders of Celltech Securities should consult their tax advisers regarding the potential application of the PFIC regime.

(iii)

Non-corporate US Holders may be subject to US federal backup withholding tax (imposed at a rate of 28 per cent.) and information reporting on payments received under the Offer if they fail to furnish and certify their correct taxpayer identification number in the manner required or otherwise fail to establish a basis for exemption. To prevent backup withholding, US Holders should complete Internal Revenue Service Form W-9 or a valid substitute form. Amounts withheld from payments to US Holders generally will be allowed as a credit against the US Holders' US federal income tax liability, provided that the required information is furnished to the Internal Revenue Service in a timely manner.

(C)  Belgian Withholding Tax

        UCB will be able to pay interest to a holder of the Loan Notes free from Belgian withholding tax so long as:

(i)

UCB receives a certificate from the holder certifying that he is the legal owner or usufructuary of the Loan Notes producing the interest, that he is a non-resident of Belgium, and does not hold the Loan Notes for the purposes of a trade or profession which he carries on in Belgium;



(ii)

the beneficial owner of the interest has been the legal owner or usufructuary of the Loan Notes during the entire period to which the interest relates;



(iii)

the Loan Notes remain in registered form; and



(iv)

UCB is a financial enterprise from the date of issue of the Loan Notes.

        For this purpose, UCB will be treated as a financial enterprise if it meets the following conditions: (1) it is a Belgian resident company or a Belgian permanent establishment of a foreign company; (2) it holds, during the tax year preceding the year during which interest is paid or a right to interest arises, shares having the nature of fixed financial assets whose investment value represents, on average, at least 50 per cent. of the total of the balance sheet at the end of the accounting year corresponding to the tax year; and (3) its shares are listed on a recognised stock exchange or it is controlled by a listed company (which means that at least 50 per cent. of its shares are directly or indirectly held by i) a Belgian resident company or by ii) a foreign company subject to corporate tax similar to the Belgian corporate tax, which does not benefit from a tax regime which is not applicable to all domestic companies or which is significantly more advantageous than the Belgian corporate tax). If UCB does not comply with these requirements in a particular accounting year, then all subsequent interest payments will be made under deduction of withholding tax (even if the requirements are satisfied at a later date), unless another exemption is available.

        If no exemption is available, payments of interest on the Loan Notes will be subject to Belgian withholding tax, currently at the rate of 15 per cent.

17

(D)  EU Directive on the taxation of savings income

        On June 3, 2003, the Council of the European Union adopted a directive on the taxation of savings income (Directive 2003/48/EC) (the "Directive") under which each Member State of the European Union ("Member State") will generally be required to provide to the tax authorities of another Member State details of payments of interest or other similar income paid by a person within its jurisdiction to or for an individual beneficiary resident in that other Member State.

        By way of exception (and for a transitional period only, which will end after agreement on exchange of information is reached between the European Union and certain non-European Union States), Belgium, Luxembourg and Austria will instead be required to impose a withholding tax, at a rate of 15 per cent. during the first three years from the date of application of the Directive, of 20 per cent. for the subsequent three years and of 35 per cent. thereafter, on such payments unless the beneficiary authorises the person making the payment to report the payment or presents a certificate from the relevant tax authority establishing exemption therefrom.

        The Directive will, subject to certain conditions being satisfied, apply from January 1, 2005.

        If and when the Directive comes into force, payments of interest on the Loan Notes made by a paying agent located in Belgium, Luxembourg or Austria to an individual beneficially entitled to the interest who is resident in