Olympic 2012 boss David Higgins leaves for Network Rail
The chief executive of the Olympic Delivery Authority is leaving to join Network Rail in a similar position.
David Higgins said that it had been a privilege to lead the organisation and added that he was confident the 2012 games would be a success.
The authority's director of finance Dennis Hone will replace Mr Higgins.
Mr Hone said there would be no change in direction or focus, saying the challenges ahead would be to hit targets and bear down on costs.
2012 chairman Lord Coe also paid tribute to Mr Higgins, praising the experience and leadership skills which he brought to the project.
He said: "David's wealth of experience and his leadership puts us in an excellent position to ensure the athletes of the world have the best possible environment to compete in 2012."
Mr Higgins will leave in February next year.
Adrian Warner, BBC Olympics correspondent said: "This is a big loss, this is a man who did all the deals and the contracts.
"When I talk to people they say he is crucial. Effectively he drove the bus, put his accelerator down and got people on the way to 2012.
"What he is saying now is, 'I'm sorry we've not got to the terminus but I'm off lads'. Inevitably that will have an impact."
His bonus will be revealed in Parliament next month
Mr Higgins said: "It has been a privilege to lead the ODA over the last five years. It is a great organisation with outstanding staff who can be justifiably proud of what they are achieving."
Mr Hone said: "This is a change of chief executive for the ODA but it does not mean a change of direction or focus.
"The challenge ahead of us is to continue to hit our milestones, bear down on costs and complete the venue construction at the same time as maintaining our excellent record on health and safety."