American-born Barbara Cassani has stood down as head of London's 2012 Olympic bid after only 11 months in the job.
In a surprise move, Cassani will be succeeded by former Olympic champion runner Sebastian Coe but will stay on as vice-chair, a post to which Coe was appointed last September.
She told a media conference on Wednesday: "I think it would be easier to sell the bid with a world champion, a double gold champion, at its head."
Cassani, 43, added: "I did not know the international sport world previously. I am getting to know it.
"I am enjoying it very much but what we need is someone who already knows it well and who is already a leader."
Her announcement came the day after London had made it along with four other cities to the International Olympic Committee's short-list to host the Games.
London 2012 organisers, announcing a "refocus of the bid leadership", said Cassani would have "specific responsibility for the development of the candidature file."
A statement said: "The changes reflect the next phase of the bid as a candidate city and the importance of giving an increased emphasis to the sporting and international aspects of the London 2012 campaign."
Businesswoman Cassani, who was appointed in June last year, was born in Boston and now lives in England with her British husband.
She was formerly head of budget airline Go, one of the travel industry's big success stories, and was voted businesswoman of the year in 2002 before being chosen last June to lead the London bid.
Cassani said: "Our focus must now be on winning the support of the international sporting community and demonstrating London is the right choice for 2012. The changes in our organisation reflect these priorities."
Cassani added: "I look back on the past 11 months with a great deal of pride. The bid puts us among the top handful of cities. I came here to win because it is the right thing for London and for sport."
Coe, 47, the 1,500 metres gold medallist at the 1980 and 1984 Olympics, is taking over as bid chairman and president.
He said: "This is a great honour and a tremendous challenge. Barbara and the team have done a fantastic job in creating the building blocks for the bid.
"Through my work in international sport, I am looking forward to promoting London's bid to the sports community across the globe."
Since his retirement as a competitive runner in 1990, Coe has concentrated on a political career. He served as an MP from 1992 to 1997 and was a close associate of former Conservative leader William Hague in his time in the House of Commons.
Since 2000, he has been a member of the upper House of Lords, sitting as Lord Coe.
John Scott, director of international relations at UK Sport, told Reuters in Lausanne: "Barbara did a great job getting us to this stage. She delivered us to candidate status but maybe now we are moving to a different phase."
"Having Seb gives us an athlete rooted in Olympism. Seb is totally dedicated to the Olympics."
He added: "Barbara brought the ability to pull together a bid. Now we need to sell it. What we are seeing is an evolution of bids. Bidding has become a very sophisticated exercise."
The vote to decide who hosts the 2012 Games is in July next year. The other cities on the short-list are Paris, Madrid, New York and Moscow.