London Mayor Boris Johnson was facing fresh embarrassment today after the resignation of a third senior aide in four months.
Tim Parker announced he was quitting as First Deputy Mayor and would not take up a job as chairman of Transport for London (TfL).
The businessman said it was "not appropriate" for him as an unelected official to carry out the role.
"I look forward to advising Boris on an ongoing basis on transport," Mr Parker said.
"I have concluded, however, that it would not be appropriate for an unelected official to chair a body which is responsible for most of the money and a large part of the brief of an elected Mayor.
"I also agree with the Mayor that my position as adviser does not justify my full time and exclusive commitment to the Greater London Authority, or the title of First Deputy Mayor."
Mr Parker had been due to take charge of TfL next month, but Mr Johnson will now retain the position of chairman.
The Mayor said: "Transport for London is responsible for a huge range of transport policies that impinge directly on the lives of Londoners and I was delighted when Tim Parker agreed to take charge of the Board on my behalf.
"Over the last few weeks, however, it has become increasingly apparent to both of us that the nature of the decisions that need to be taken are highly political and there is no substitute for me, as the directly elected Mayor, being in charge."
The loss of Mr Parker follows the departure of Ray Lewis, another deputy mayor, over allegations of impropriety, and James McGrath, a senior adviser, for comments he made about immigrants.
But Mr Johnson insisted London had not "lost the services" of Mr Parker, as he would continue to advise the administration.
Opposition parties had previously criticised the Mayor for delegating so much power to one of his appointees.
Mike Tuffrey, the leader of the Liberal Democrat group in the London Assembly, said: "To lose one advisor is unfortunate, to lose two is careless but to lose three in four months shows the wheels are coming off this new administration.
"Why is Boris losing yet another advisor? Has Tim Parker discovered that running London isn't as easy as running private business?"