Boris Johnson said he was backing David Cameron "all the way" and was "increasingly confident" the Prime Minister would win the 2015 election for the Tories, amid fresh claims about his party leadership ambitions.
Ex-Downing Street spin chief Andy Coulson reignited speculation by claiming the London Mayor was desperate to be prime minister but would rather see David Cameron "fail miserably" and take his place than stab him in the back.
"Grateful as I always am to Andy for career advice, I am backing David Cameron all the way," Mr Johnson declared as he attended an education summit in central London.
"I'm backing David who I am absolutely increasingly confident is going to win," he added.
Mr Coulson, the former News of the World editor who is due to stand trial later this year over allegations related to phone-hacking, wrote in GQ magazine that the mayor's preferred strategy would be to "ride in on his bike to save party and country" after a Tory defeat at the ballot box.
"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, controversially appointed by Mr Cameron as 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".
And on the shadow cabinet, he said its members appeared "detached" from Mr Miliband.
"I just don't think they rate him very much," he said.
"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.