Lemley's contract was due to run until March 2010

The American, who ran the Anglo-French group which designed and built the Channel Tunnel, says he wants to spend more time on his construction business.

Civil Aviation Authority chairman Sir Roy McNulty has been given the role in a temporary capacity.

"It has been a privilege to chair the ODA and lead the organisation through its formative stages," said Lemley.

The ODA is the body responsible for delivering the venues and infrastructure for the Olympic and Paralympic Games in London in 2012.

Lemley was appointed as chairman in November last year but officially took the role in March.

His contract was due to run until March 2010.

"Jack has played a significant role in getting us to this point and helping ensure that Games and legacy are planned together," said Olympics minister Tessa Jowell.

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 jack Lemley

"I am grateful to him for all he has done and wish him well in the future."

Lemley added: "The board is now established along with its separate planning function. The delivery partner has been appointed and the ODA is ready to move to the next stage of its development.

"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."