The agency already handled advertising for the Lunn Poly brand, but the formal appointment is a blow to HHCL & Partners, which has handled advertising for Thomson since 1998.
TUI called a pitch through the AAR at the beginning of the year. HHCL, Abbott Mead Vickers BBDO and Mustoe Merriman Levy, the incumbent on the Skytours account, were shortlisted, along with WCRS.
However, the pitch fizzled out after the company's German parent, TUIAG, decided to embark on its own marketing review. WCRS started working on the Thomson business after the UK pitch was abandoned.
Stephen Woodford, the chief executive of WCRS, said: "The TUI Group is the most exciting and dynamic travel company in Europe, and I am delighted to be able to build on the success of our relationship with Lunn Poly to work across all the TUI brands."
The agency will kick off its advertising for both the Lunn Poly and Thomson brands in August with a major campaign to launch the group's summer programme for 2003.
A recent campaign by HHCL for Thomson had adopted a costume drama theme which had featured the fictitious lovers Daversham and Fanny.
Last August, the German company Preussag bought Thomson Travel Group - then the UK's largest package holiday company - for £1.8 billion, to create Preussag-Thomson. It then launched the TUI parent brand in October 2001 in a bid to unite the various brands of TUI and the Thomson Travel Group.