David Bedford, the former athlete in charge of the Olympics marathon, has resigned after a row with 2012 organisers.
Mr Bedford quit after a series of bust-ups with the London Organising Committee. He said: "It's very difficult to work for another organisation who have never organised an event of their own."
London Marathon chief executive Nick Bitel said Bedford's decision was a personal one and the organisation would continue to work closely with Games organisers on the marathons and race walk events.
Bitel told Press Association Sport: "David has decided not to work on the Olympic events any more. That's his personal choice and I very much regret his decision.
"The decision was due to his frustration with LOCOG's (London Organising Committee of the Olympic Games) approach to event management."
Bitel would not expand on what had caused the frustration but said Bedford would be missed.
He added: "He has incredible knowledge and experience so we regret the fact he's not working on the events, but we are continuing to work closely and harmoniously with LOCOG and we are confident that they will be great events."
Bedford himself told the Evening Standard: "I quit my role; I resigned. We (London Marathon) are a professional business run by professionals and it was very difficult to work for another organisation who in the main have never organised an event of their own."
Bedford also described the handling of the decision to cut out East End boroughs from the marathon route as "appalling", according to the Standard.
One of the issues reported to have caused Bedford to quit was the test event for the marathon in May.
Bitel said, however, they were more than happy with the arrangements for the testing.
A London 2012 spokeswoman said: "This was Dave Bedford's own decision. It hasn't affected the strong working relationship that the London Marathon has with LOCOG.
"Both organisations are working closely together on the test event in May 2011, and on the delivery of first-class Olympic and Paralympic marathons in 2012."