Sir Roy McNulty, the ODA Board’s deputy chairman, will lead the organisation as acting chairman, working alongside David Higgins, chief executive, until a decision is made about a permanent successor.

 

Lemley suffered a heart scare earlier in the year and is understood to have had a minor stroke. "His decision today has nothing to do with his health," said a spokesman for the ODA.

 

But London Assembly chairman Brian Coleman claimed Lemley's health had played a part in his decision. "I don't know why it took him so long to resign,'' he said. "This is an opportunity to appoint a strong candidate to get the Olympics back on track.''

 

Coleman, a Conservative, said that Lemley's successor should be a Briton. Higgins is Australian.

 

Lemley, a former sergeant in the United States Army, offered no reason for his departure beyond saying in the ODA statement that his construction-consulting business, based in Boise, Idaho, had so much work on.

 

Lemley was appointed chair of the Olympic Delivery Authority in November 2005. The ODA is the single delivery body responsible for creating the venues and infrastructure for the Olympic Games and Paralympic Games in London in 2012. 

 

"It has been a privilege to chair the ODA for the last year and lead the organisation through its formative stages," said Lemley. "The Board is now established, along with its formative stages. The Delivery Partner has been appointed and the ODA is ready to move to the next stage of its development."

 

From 1989 to 1993 Lemley was chief executive of Transmanche-Link, the consortium of 10 British and French companies responsible for constructing and equipping the Channel Tunnel, the largest privately-funded construction project in history.

 

Since then he has run his own consultancy in the US with many high-profile international contracts, including projects in Turkey, Cyprus and Venezuela.

 

"I am keen to return to the helm of my international construction consulting firm in America, which is increasingly busy with major contracts," he said.

 

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

 

Olympics Minister Tessa Jowell said:  "Jack has done much to help lay the foundations for success for the Olympic construction project over the last year. The ODA has made a strong strong to its programme. The final site map for the Olympic Park has been published along with the timetable for the construction programme.

 

"With the Delivery Partner on board we now move to the next stage of the project - venue and infrastructure design. 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."

 

Ken Livingstone, the Mayor of London, said: "Jack has played an important role in helping London’s transition from CandidateCityto host city for the 2012 Olympic Games and Paralympic Games over the last year. He has given leadership and direction to the ODA during this period. I respect his decision to return to the USat this stage of the project and wish him every success.’

Sebastian Coe, chairman of the London 2012 Organising Committee, said: "Jack has been instrumental in defining the design and construction timelines that the Olympic Games and Paralympic Games project is based on. 

 

"His creation of the 2-4-1 programme has driven the planning to date and helped finalise the Olympic Park plan six years before the Games, which has given us clarity and confidence to move to the next stage of designing and constructing the venues."

 

Sir Roy has been chairman of the Civil Aviation Authority since 2001. Previously he was chairman of National Air Traffic Services (NATS) from 1999 to 2001, and prior to that he was chief executive and latterly chairman of Shorts Brothers plc, the Belfast-based aerospace company.