The American businessman quit overnight, less than a year after being appointed on a four-year contract which was due to run until March 2010.

The the head of the body responsible for delivering the venues and Games infrastructure, will now concentrate on his US construction interests.

Sir Roy McNulty, chairman of the Civil Aviation Authority since 2001, has been made acting chairman in his place.

Olympics chiefs are understood to be disappointed Lemley has left after such a short time.

Lemley, who ran the Anglo-French group that designed and built the Channel Tunnel, said in a statement: "I am keen to return to the helm of my international construction consulting firm in America, which is increasingly busy with major contracts.

"I have every confidence that London will stage a superb Olympic and Paralympic Games in 2012 and leave a legacy that the country can be proud of."

Lemley was appointed in November 2005 but only officially assumed the chairmanship in March.

In a statement, Olympics minister Tessa Jowell said Lemley had done much to lay the foundations.

She said: "Jack has played a significant role in getting us to this point and helping ensure that Games and legacy are planned together. I am grateful to him for all he has done and wish him well in the future."

Seb Coe, chairman of the London organising committee said: "Jack has been instrumental in defining the design and construction timelines that the Olympic and Paralympic Games project is based on."