Hammersley, who took the top job at DDB in March 2004, will become a partner in the new agency. He is leaving DDB immediately.
Michael Bray, the president of DDB Europe, will take over as DDB London's acting chief executive.
Bray said: "While we are sorry to see Paul leave, we do understand his decision to want to explore new challenges. We'd like to thank him for all the work he's done at DDB. He leaves behind an agency in great shape and poised for a terrific 2006. We wish him every success in his new venture."
Hammersley added: "This is an opportunity which I can not pass up; an opportunity to be part of a partnership which will return creativity to the top of the agenda."
It is uncertain whether David Hackworthy, DDB's chief strategic officer, will also leave the agency. Hammersley recruited Hackworthy in August 2004 and the pair were expected to join the new venture together. DDB said it did not "envisage any further management changes".
Sir Frank announced the launch of his start-up and the capture of Lowe's £50m Tesco advertising account a week ago.
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