Boris Johnson is desperate to be prime minister but would rather see David Cameron "fail miserably" at the next election than stab him in the back, former government spin doctor Andy Coulson has said.
The ex-News Of The World editor said the mayor of London's preferred strategy would be to "ride in on his bike to save party and country" should Mr Cameron and the Conservatives lose the 2015 general election.
Mr Coulson, former No10 director of communications, advised Downing Street to support Mr Johnson's good ideas, "advise privately on the bad ones, but only engage publicly if absolutely necessary - and celebrate Boris's considerable successes".
Writing in GQ, he said it would be "off brand" for Mr Johnson to plot against the Prime Minister.
"Boris Johnson desperately wants to be prime minister and David has known that fact longer than most," Mr Coulson wrote.
"When Boris asked me to pass on the message that he was keen to stand as mayor of London, David responded 'Well, if he wins, he'll want my job next'."
Mr Coulson went on: "If proof were needed that our PM is a man untroubled by self doubt, it came in his next sentence - 'so I think he'll be a bloody brilliant candidate for us'.
"Stabbing David, or anyone else for that matter, in the back would be distinctly off brand - just not very Boris.
"He would much prefer to see David fail miserably in the election and ride in on his bike to save party and country."
Mr Coulson, who is due to stand trial later this year over allegations related to phone-hacking, said the Tories' strategy for beating Labour should involve exposing leader Ed Miliband's strategy of "keeping his head down" and "pray Ed Balls remains shadow chancellor until the election".
On Mr Miliband, he wrote: "The prime minister must push him to take positions: expose his strategy (to keep his head down, silently hope that the economy continues to go wonky and, well, just be the other guy), challenge him to take a view on the tricky issues opposition politicians love to duck....
"I'm struck by how detached the opposition front bench appears to be from their leader... I just don't think they rate him very much.
"And if they don't, there's a good chance the public will feel the same way once they get to know him properly."
He likened the relationship between Mr Miliband and Mr Balls to the "shamefully dysfunctional" Tony Blair and Gordon Brown partnership when Labour were in power.
"The prime minister should pray Ed Balls remains shadow chancellor until the election," Mr Coulson wrote.
"Appointing him as George's opposite number was the Miliband gift that will keep on giving...
"The Tories must look for the divisions and make the most of them a) because they are most certainly real - always a plus - and b) because it's history repeating itself.
"We are in this hole at least in part because of the shamefully dysfunctional Blair/Brown relationship.
"Labour's Two Eds dislike each other and each thinks he is smarter than the other.
"The Conservatives should imagine in some detail how it would work if they actually won...and share that vision with the British public."
Mr Coulson is also facing charges over an alleged conspiracy to bribe public officials for information.