EXHIBIT 99.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350 AS ADOPTED PURSUANT TO SECTION
906 OF THE SARBANES-OXLEY ACT OF 2002

        In connection with the Annual Report of Willis North America Financial
Security Partnership Plan (the "Plan") on Form 11-K for the year ended December
31, 2002, as filed with the Securities and Exchange Commission on the date
hereof (the "Report"), I, Joseph J. Plumeri, Chairman and Chief Executive
Officer of Willis Group Holdings Limited, pursuant to 18 U.S.C. ss.1350, as
adopted pursuant to ss.906 of the Sarbanes-Oxley Act of 2002, certify that:

(1) The Report fully complies with the requirements of Section 13(a) or 15(d) of
the Securities Exchange Act of 1934; and

(2) The information contained in the Report fairly presents, in all material
respects, the net assets available for benefits and changes in net assets
available for benefits of the Plan.

                                        By: /s/ JOSEPH J. PLUMERI
                                            ------------------------------------
                                            Joseph J. Plumeri
                                            Chairman and Chief Executive Officer

Date: June 27, 2003

        A signed original of this written statement required by Section 906 has
been provided to Willis Group Holdings Limited and will be retained by Willis
Group Holdings Limited and furnished to the Securities and Exchange Commission
or its staff upon request.