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                                                                    EXHIBIT 12.1

            CERTIFICATION OF PETER CHAMBRE PURSUANT TO SECTION 302 OF
                         THE SARBANES-OXLEY ACT OF 2002

     I, Peter Chambre, certify that:

     1.    I have reviewed this annual report on Form 20-F/A of Cambridge
Antibody Technology Group plc;

     2.    Based on my knowledge, this report does not contain any untrue
statement of a material fact or omit to state a material fact necessary to make
the statements made, in light of the circumstances under which such statements
were made, not misleading with respect to the period covered by this report;

     3.    Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material respects the
financial condition, results of operations and cash flows of the company as of,
and for, the periods presented in this report;

     4.    The company's other certifying officer and I are responsible for
establishing and maintaining disclosure controls and procedures (as defined in
Exchange Act Rules 13a-15(e) and 15d-15(e)) for the company and have:

     (a)   Designed such disclosure controls and procedures, or caused such
disclosure controls and procedures to be designed under our supervision, to
ensure that material information relating to the company, including its
consolidated subsidiaries, is made known to us by others within those entities,
particularly during the period in which this report is being prepared;

     (b)   Evaluated the effectiveness of the company's disclosure controls and
procedures and presented in this report our conclusions about the effectiveness
of the disclosure controls and procedures, as of the end of the period covered
by this report based on such evaluation; and

     (c)   Disclosed in this report any change in the company's internal control
over financial reporting that occurred during the period covered by the annual
report that has materially affected, or is reasonably likely to materially
affect, the company's internal control over financial reporting; and

     5.    The company's other certifying officer and I have disclosed, based on
our most recent evaluation of internal control over financial reporting, to the
company's auditors and the audit committee of the company's board of directors
(or persons performing the equivalent function):

     (a)   All significant deficiencies and material weaknesses in the design or
operation of internal control over financial reporting which are reasonably
likely to adversely affect the company's ability to record, process, summarize
and report financial information; and

     (b)   Any fraud, whether or not material, that involves management or other
employees who have a significant role in the company's internal control over
financial reporting.

                                                        Date:  October 14, 2003

                                    /s/ Peter Chambre
                                    --------------------------------------------
                                    Name:  Peter Chambre
                                    Title: Chief Executive Officer