<PAGE>


                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

                                   -----------
                                   FORM 20-F/A
                                Amendment No. 1

(Mark One):
[_] Registration statement pursuant to Section 12(b) or 12(g) of the Securities
    Exchange Act of 1934; or
[X] Annual report pursuant to Section 13 or 15(d) of the Securities Exchange Act
    of 1934
        For the fiscal year ended September 30, 2001; or
[_] Transition report pursuant to Section 13 or 15(d) of the Securities Exchange
    Act of 1934
        For the transition period from _________ to _________
                        Commission file number 000-31116

                     CAMBRIDGE ANTIBODY TECHNOLOGY GROUP PLC

             (Exact name of registrant as specified in its charter)

                                England and Wales
                 (Jurisdiction of Incorporation or Organization)

                                The Science Park
                        Melbourn, Cambridgeshire England SG8 6JJ
                    (Address of Principal Executive Offices)

        Securities to be registered pursuant to Section 12(b) of the Act:
                                      None

        Securities to be registered pursuant to Section 12(g) of the Act:

                           American Depositary Shares
                                (Title of Class)

 Securities for which there is a reporting obligation pursuant to Section 15(d)
                                  of the Act:
                                      None

Indicate the number of outstanding shares of each of the issuer's classes of
capital or common stock as of the close of the period covered by the annual
report.

          American Depositary Shares: 355,553 as of September 30, 2001
              Ordinary Shares: 35,455,865 as of September 30, 2001

Indicate by check mark whether the registrant: (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days:

                 Yes   X        No_________     Not Applicable ____
                     -----

Indicate by check mark which financial statement item the registrant has elected
to follow:

                          Item 17 ___     Item 18  X
                                                  ---

<PAGE>

     Pursuant to Rule 12b-23(a) of the Securities Exchange Act of 1934, as
amended, the information for the Annual Report on Form 20-F of Cambridge
Antibody Technology Group plc (the "Company") set out below is being
incorporated by reference from the Company's "Annual Report and Accounts and
Form 20-F for the year ended September 30, 2001" filed as Exhibit 99.1 to the
Company's Report on Form 6-K dated December 31, 2001 and attached as Exhibit
99.1 to this Form 20-F (the "Annual Report").

     References below to major headings include all information under such major
headings, including subheadings, unless such reference is part of a reference to
a subheading, in which case such reference includes only the information
contained under such subheading.

     The information set forth under the heading "Forward Looking Statements" in
Note 28 to the financial statements included in the Annual Report is
incorporated herein by reference.

                                     PART I

ITEM 1 - IDENTITY OF DIRECTORS, SENIOR MANAGEMENT AND ADVISERS

     Not applicable.

ITEM 2 - OFFER STATISTICS AND EXPECTED TIMETABLE

     Not applicable.

ITEM 3 - KEY INFORMATION

     A.   Selected Financial Data

     The information set forth under the headings "Exchange Rate Information" in
Note 28 to the financial statements in and "Selected Financial Data" in the
Annual Report is incorporated herein by reference.

     B.   Capitalization and Indebtedness

     Not applicable.

     C.   Reason for the Offer and Use of Proceeds

     Not applicable.

     D.   Risk Factors

     The information set forth under the heading "Risk factors" contained in
Note 28 to the financial statements included in the Annual Report is
incorporated herein by reference.

ITEM 4 - INFORMATION ON THE COMPANY

     A.   History and Development of the Company

     The information set forth under the heading "The Company" in Note 28 to the
financial statements, the heading "Company Secretary and Registered Office" and
in the second paragraph under the heading "Liquidity and Capital Resources" in
the Annual Report is incorporated herein by reference.

                                       2

<PAGE>

     B.   Business Overview

     The information set forth under the headings "Chairman's Statement",
"Review of the Year" (including information under the subheadings beginning with
"Company expertise" through and including "Significant corporate milestones"),
Note 2 to the financial statements, "Turnover and ordinary activities before
taxation" (the second table only) and "Material Contracts" in Note 28 to the
financial statements in the Annual Report is incorporated herein by reference.

     In January 2001 the Company entered into an alliance with Elan Pharma
International Limited ("Elan") under which the companies will jointly discover
and develop antibody-based therapeutics targeted at a variety of neurological
diseases and disorders. Under the terms of the four year agreement, the two
companies will fund all research and development jointly and share
commercialization profits equally. The Company will have primary responsibility
for the discovery and optimalization of human antibodies against multiple
targets identified by Elan. Elan will evaluate clinical antibody candidates in
pre-clinical models, manage clinical and regulatory development and will be
responsible for global commercialization. Either party may terminate the
agreement for convenience on 60 days' notice. If the agreement is terminated for
convenience, then the non-terminating party will have the right to continue to
develop and commercialize products developed in the collaboration and would be
required to pay the terminating party (a) a royalty on sales of those products
and (b) a milestone payment of between US$5 million and US$15 million when
those products receive regulatory approval. The amount of the royalty and
milestone would depend on the stage of development of those products achieved at
the date of termination.

     Recent Developments

     Offer to Purchase Shares of Drug Royalty Corporation Inc.

     The Company and Drug Royalty Corporation ("DRC"), a Canada-based company
whose shares are listed on the Toronto Stock Exchange and whose assets primarily
consist of a portfolio of investments in the global healthcare market consisting
of royalty streams generated from participants in the healthcare sector, entered
into a definitive agreement on January 16, 2002 pursuant to which the Company
made an offer by way of a takeover bid for all of the issued shares of DRC.
Formal offer documentation was mailed to DRC's shareholders on February 4, 2002.
The initial expiry date of the offer is March 12, 2002, unless the offer is
extended.

     DRC's board of directors has recommended that DRC's shareholders accept the
offer. Certain DRC shareholders, holding in aggregate approximately 30% of DRC's
fully diluted share capital, have agreed to tender their shares to the offer.

     Under the terms of the definitive agreement, the Company offered DRC
shareholders C$3.00 per DRC share in its shares or American Depositary Shares
("ADSs"). The number of shares or ADSs to be paid to DRC's shareholders will be
determined by a formula that divides C$3.00 by the volume weighted average of
the trading prices of the shares on the London Stock Exchange for the ten
trading days randomly selected from the fifteen trading days ending four days
immediately prior to the initial expiry date, converted into Canadian dollars at
a pound sterling/Canadian dollar exchange rate based on the average noon spot
exchange rate for the same ten trading days, subject to a minimum exchange ratio
of 0.063. This ratio will be announced upon its determination four days
immediately prior to the initial expiry date of the offer and will thereafter
remain fixed including for any extensions of the offer.

     If the share exchange ratio is greater than 0.076 but less than or equal to
0.087, the Company will pay the amount of consideration attributable to such
excess in either or any combination it elects of cash, shares or ADSs (depending
on the form of consideration chosen by the shareholder). If the share exchange
ratio exceeds 0.087, the Company will not be required to complete the
transaction. The Company in such circumstance may nevertheless choose at the
time the share exchange ratio is announced to complete the transaction.

     The offer is subject to other customary conditions for a Canadian
transaction of this type, including a minimum level of acceptances of 66 2/3% of
the outstanding shares of DRC and the admission of the new shares to be issued
pursuant to the offer to the Official List of the United Kingdom Listing
Authority and to trading on the London Stock Exchange.

     The Company's offer values DRC at approximately C$126.2 million
((pound)55.0 million) on a fully diluted basis.

     The Company and DRC also agreed to amend their royalty agreement (described
in "Material Contracts" in Note 28 to the financial statements in the Annual
Report) to permit the Company to

                                       3

<PAGE>

terminate the agreement upon payment to DRC of C$14 million ((pound)6.1 million)
in cash or shares or a combination of shares and cash at the Company's election,
in certain circumstances in the event of a change of control of DRC.

     MorphoSys Litigation

     In MorphoSys' patent proceedings relating to the Company's Griffiths patent
pending in the U.S. District Court in Washington D.C., the court ruled on
December 21, 2002, that it intended to rule that MorphoSys did not infringe the
Company's Griffiths patent unless the court was persuaded by January 15, 2002
that there was a genuine issue of any material fact which would require the
matter to be retired before a jury. The Company filed further submissions on
this point prior to the January 15 deadline and MorphoSys responded on January
28, 2002. The parties are currently awaiting the court's decision.

     C.   Organizational Structure

     The information set forth in the second table under Note 11 to the
financial statements, "Fixed asset investments" and under "Basis of
Consolidation" under Note 1 to the financial statements, "Accounting policies"
in the Annual Report is incorporated herein by reference.

     D.   Property, Plants and Equipment

     The information set forth in the second paragraph under the heading
"Significant corporate milestones", in the second paragraph under the heading
"Liquidity and capital resources" and under the heading "Facilities" in Note 28
to the financial statements in the Annual Report is incorporated herein by
reference.

ITEM 5 - OPERATING AND FINANCIAL REVIEW AND PROSPECTS

     The information set forth under the heading "Financial Review" in the
Annual Report is incorporated herein by reference.

ITEM 6 - DIRECTORS, SENIOR MANAGEMENT AND EMPLOYEES

     A.   Directors and Senior Management

     The information set forth under the headings "Board of Directors", "Senior
Executives", and "Scientific Advisory Board" in the Annual Report is
incorporated herein by reference.

     B.   Compensation

     The information set forth under the heading "Remuneration Report" in the
Annual Report is incorporated herein by reference.

     C.   Board Practices

     The information set forth under the headings "Board of Directors",
"Remuneration Report--Notice Periods", "Corporate Governance--The Board of
Directors", "Corporate Governance--The Audit Committee" and "Corporate
Governance--The Remuneration Committee" is incorporated herein by reference.

                                       4

<PAGE>

     D.   Employees

     The information set forth in the first table under Note 6 to the financial
statements, "Staff Costs" in the Annual Report is incorporated herein by
reference.

     E.   Share Ownership

     The information set forth under the headings "Remuneration
Report--Directors' Share Options", "Remuneration Report--Directors'
Shareholdings", "Remuneration Report--Company Share Option Plan", "Remuneration
Report--All-Employee Share Option Plan", Note 6 to the financial statements,
"Staff Costs" and Note 17 to the financial statements, "Called up share capital
and share premium" (in the options table) in the Annual Report is incorporated
herein by reference.

ITEM 7 - MAJOR SHAREHOLDERS AND RELATED PARTY TRANSACTIONS

     The information set forth under the heading "Directors' Report--Substantial
Shareholdings" and Note 25 to the financial statements, "Related party
transactions" in the Annual Report is incorporated herein by reference.

ITEM 8 - FINANCIAL INFORMATION

     The information set forth under the headings "Independent Auditors'
Report", "Consolidated Profit and Loss Account", "Consolidated Statement of
Total Recognized Gains and Losses", "Consolidated Balance Sheet", "Company
Balance Sheet", "Consolidated Cash Flow Statement" and "Notes to Financial
Statements" in the Annual Report is incorporated herein by reference.

ITEM 9 - THE OFFER AND LISTING

     The information set forth under the heading "Market for CAT's Ordinary
Shares" in Note 28 to the financial statements in the Annual Report is
incorporated herein by reference.

     The high and low prices for the shares of the Company trading on the London
and Nasdaq Stock Exchanges for November and December 2001 and January 2002 is
set out below.

                                          Ordinary LSE            ADS Nasdaq
                                     ----------------------  -------------------
                                        High        Low         High      Low
                                       (pence)    (pence)      (US$)     (US$)
                                     ----------  ----------  --------- ---------
        2001          -November         1998       1650        28.82     23.15
                      -December         1973       1560        26.75     22.75
        2002          -January          1949       1403        27.25     22.07

ITEM 10 - ADDITIONAL INFORMATION

     A.   Share Capital

     Not applicable.

                                       5

<PAGE>

     B.   Memorandum and Articles of Association

     The information set forth under the heading "Description of Share Capital"
in the Company's Registration Statement on Form 20-F/A, as amended, dated June
5, 2001, is incorporated herein by reference.

     C.   Material Contracts

     The information set forth under the heading "Material Contracts" in Note 28
to the financial statements in the Annual Report is incorporated herein by
reference.

     D.   Exchange Controls

     None.

     E.   Taxation

     The information set forth under the headings "Taxation" in Note 28 to the
financial statements and "PFIC Annual Information Statement" in the Annual
Report is incorporated herein by reference.

     F.   Dividends and Paying Agents

     Not applicable.

     G.   Statement by Experts

     Not applicable.

     H.   Documents on Display

     The exhibits to this Form 20-F are available for review at the Company's
registered office.

     I.   Subsidiary Information

     Not applicable.

ITEM 11 - QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

     The information set forth in Note 16 to the financial statements,
"Financial Instruments" in the Annual Report is incorporated herein by
reference.

ITEM 12 - DESCRIPTION OF SECURITIES OTHER THAN EQUITY SECURITIES

     Not applicable.

                                    PART II

ITEM 13 - DEFAULTS, DIVIDEND ARREARAGES AND DELINQUENCIES

     (a)  There has been no material default in payment of principal, interest,
a sinking or purchase fund installment, or any other material default with
respect to any indebtedness of the Company or any of its significant
subsidiaries.

                                       6

<PAGE>

     (b)  There have been no arrears in the payment of dividends on, and no
material delinquency with respect to, any class of preferred stock of any
significant subsidiary of the Company.

ITEM 14 - MATERIAL MODIFICATIONS TO THE RIGHTS OF SECURITY HOLDERS AND USE OF
PROCEEDS

     Not  applicable.

                                    PART III

ITEM 17 - FINANCIAL STATEMENTS

     The Company has responded to Item 18 in lieu of this item.

ITEM 18 - FINANCIAL STATEMENTS

     The information set forth under the headings "Independent Auditors'
Report", "Consolidated Profit and Loss Account", "Consolidated Statement of
Total Recognized Gains and Losses", "Consolidated Balance Sheet", "Company
Balance Sheet", "Consolidated Cash Flow Statement" and "Notes to Financial
Statements" in the Annual Report is incorporated herein by reference.

ITEM 19 - EXHIBITS

     Please see the attached "Index to Exhibits".

                                       7

<PAGE>

     The registrant hereby certifies that it meets all of the requirements for
filing on Form 20-F and that it has duly caused and authorized the undersigned
to sign this registration statement on its behalf.

                                      CAMBRIDGE ANTIBODY TECHNOLOGY GROUP PLC



                                      By: /s/ John Aston
                                         ---------------------------------------
                                      Name:  John Aston
                                      Title: Chief Financial Officer
                                      Date:  October 14, 2003

                                       8

<PAGE>

                                INDEX TO EXHIBITS

     Exhibit   Description
     -------   -----------

     1.1       Memorandum and Articles of Association*

     2.1       Specimen of ordinary share certificate*

     2.2       Form of Deposit Agreement*

     2.3       Form of ADR*

     +4.1      License Agreement, dated as of January 7, 1997, between CAT and
               the Medical Research Council*

     4.2       Lease Agreement, relating to the lease of Beech House*

     4.3       Lease Agreement, relating to the lease of a building in South
               Cambridgeshire, England*

     4.4       Lease Agreement, relating to facilities to be constructed in the
               future*

     +4.5      Development and License agreement, dated April 1, 1995 by and
               between Cambridge Antibody Technology Limited and Knoll AG*

     +4.6      Development and License Agreement, dated February 28, 1999,
               between Knoll AG and Cambridge Antibody Technology Limited*

     +4.7      License and Collaboration Agreement, dated as of September 27,
               2000, between Cambridge Antibody Technology Limited and Genzyme
               Corporation*

     +4.8      Collaboration Agreement, dated August 9, 1999, between Human
               Genome Sciences, Inc. and Cambridge Antibody Technology Limited*

     +4.9      Antibody License Agreement, dated February 29, 2000, between
               Human Genome Sciences, Inc. and Cambridge Antibody Technology
               Limited*

     +4.10     Research Collaboration and Service Agreement, dated March 29,
               1999, by and among Genetics Institute, Inc. and Cambridge
               Antibody Technology Limited*

     +4.11     Research and License Agreement, dated December 23, 1999, by and
               between G.D. Searle & Co. and Cambridge Antibody Technology Group
               plc*

     +4.12     Form of Executive Director Service Agreement*

     +4.13     Form of Non-executive Director Service Agreement*

     +4.14     Research, Development and Commercialization Agreement, dated
               January 24, 2001, between Elan Pharma International Limited and
               Cambridge Antibody Technology Limited

                                       9

<PAGE>

     8.1       List of subsidiaries.  The information required by this exhibit
               is contained in Note 11 to the financial statements, "Fixed asset
               investments" in the Annual Report and is incorporated herein by
               reference.

    12.1       Certification of Peter Chambre pursuant to Section 302 of the
               Sarbanes-Oxley Act of 2002

    12.2       Certification of John Aston pursuant to Section 302 of the
               Sarbanes-Oxley Act of 2002

    13.1       Certification of Peter Chambre pursuant to Section 906 of the
               Sarbanes-Oxley Act of 2002

    13.2       Certification of John Aston pursuant to Section 906 of the
               Sarbanes-Oxley Act of 2002

    99.1       Annual Report for the Year Ended September 30, 2001,
               (previously filed).

*    Filed with the Company's Registration Statement on Form 20-F/A, dated
June 5, 2001 under the same exhibit number.

+    Confidential treatment requested for portions of this exhibit.

                                       10