Labour MP and former sports minister Kate Hoey will work for Boris Johnson if he becomes London's mayor in Thursday's election.
The Conservatives said Hoey, a critic of Livingstone's administration, would be an unpaid non-executive director advising on sport and the 2012 Olympics.
Speculation that Hoey could back the Tory candidate has been rife since she was scheduled to appear alongside him at a campaign event last week but pulled out, blaming ill health.
Sources within the campaign said they did not expect her to quit as a Labour MP and Johnson said he was pleased to be constructing a potential administration from "across politics".
Johnson said: "I am delighted to announce that Kate Hoey will join me in my administration if I win on May 1.
"She and I agree that there is much more that can be done to promote sport and to develop sporting facilities across London.
"I have already pledged to ring-fence London Development Agency money for sport, and if I am lucky enough to win she will be working on an agenda that includes protecting playing fields, boosting sports clubs and making sure that London's kids all benefit from the Olympics."
He went on: "Kate has a huge and well-known commitment to sport and to London, and I am determined to bring talent from across politics and the community to a new administration."
Hoey complained in January about a lack of scrutiny of Livingstone's City Hall advisers and a "cult of silence" which stopped people speaking out about alleged malpractice.
"There has been a cult of silence because... every time anyone raises a criticism you are either branded as being racist or being pro-Boris Johnson," she said.

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