The company has appointed Agency Assessments International to conduct the search. The incumbent agency, DDB London, will repitch for the business.
Andrew Rayner, the Thomson marketing director, said: "Thomson is at an important crossroads in its development, as it moves from a traditional holiday company to a full-service travel portal. This will be a substantial challenge, and we want to ensure we have the most effective strategy and creative execution in place, to achieve this repositioning.
"We have been working with DDB London since April 2003 and it has added tremendous value to the accounts, and hence will be part of the pitch process."
The pitch does not affect the media account, held by Media Planning Group. Thomson's UK direct marketing is currently being reviewed.