Jim Thornton is quitting Leo Burnett without a job to go to after more than four years as its executive creative director.
Thornton, who will not be directly replaced, is putting together plans for a start-up, that he says will be a "non-traditional ad agency".
Bruce Haines, the chairman of the Leo Burnett UK group, is now looking for an executive creative director who will have responsibility for the ad agency and Arc London.
Until a new appointment is made, Mark Tutssel, Leo Burnett Worldwide's chief creative officer, will assume creative responsibility for the London agency.
During Thornton's tenure in charge, the department won numerous awards for clients, including the "spiderman" poster for Heinz, and the "camera phone" teen road-safety TV spot for the Department for Transport.
He was also able to attract some heavyweight creative talent, including the HHCL creative directors, Jonathan Burley and Jim Bolton, who joined in January 2006.
However, during his time in charge, the agency also failed to convert on a number of major pitches, the largest and most high profile being the 2005 Sainsbury's review.
Both Haines and Thornton said the split was amicable. Haines added: "He joined us because he wanted to prove that you could do great work on big mainstream brands; he's done that. He has also provided a new creative spark. He is a brave and committed leader, and always pushes us to consider the impossible."
Thornton joined Burnett from Mother in 2003 with Paul Shearer from Wieden & Kennedy Amsterdam. They became joint executive creative directors, replacing Nick Bell, who had left to take on the lead creative role at JWT. Shearer departed in September 2004 to join Nitro.
Before Thornton started at Mother in 1999, he spent three years at TBWA\GGT. Before that, he worked at JWT as a copywriter for ten years.
Under Thornton's stewardship, Leo Burnett became the first London agency to set up a creative department in Second Life.