London Mayor Boris Johnson wants David Cameron to "fail miserably", says Andy Coulson, the Prime Minister's former media chief.
Tory defeat at the next general election in 2015 would almost certainly lead to Cameron's resignation, opening the way for Johnson to return as an MP, after his mayoralty ends in 2016, and take over as party leader.
"Boris Johnson desperately wants to be Prime Minister and David has known that fact longer than most," said Coulson in an interview with GQ magazine.
On the plus side for Cameron, Johnson's tactics as outlined by Coulson would see loyal behaviour out of City Hall for the next two years.
"Stabbing David in the back would be distinctly off brand, just not very Boris," said Coulson, forced to resign his Director of Communications job at No. 10 Downing St over the phone-hacking scandal.
"He would much prefer to see David fail miserably in the election and ride in on his bike to save party and country."
Johnson has admitted he fancies a go at the top job, using a mangled rugby metaphor to illustrate the point.
"If the ball came loose from the back of the scrum, which it won't of course, it would be a great, great thing to have a crack at," he said earlier this year.