By Andrew Osborn
LONDON (Reuters) - British Prime Minister David Cameron's position is under threat from the mayor of London, who is waiting for him to "fail miserably" at the next election so he can take his job, the leader's former communications chief has said.
In a rare foray into politics, Andy Coulson said he thought Boris Johnson, the British capital's flamboyant mayor, was playing a waiting game, hoping Cameron would lose the 2015 election so that he could assume his role as leader of the Conservative party.
"Stabbing David, or anyone else for that matter, in the back would be distinctly off brand - just not very Boris," Coulson wrote in GQ magazine in an article to be published on Thursday.
"He would much prefer to see David fail miserably in the election and ride in on his bike to save party and country," he added, referring to Johnson's preferred mode of transport.
Known for his colorful turn of phrase and unruly mop of blond hair, Johnson, 48, was credited with playing a key role in last year's successful London Olympics, and supporters say he has the electoral appeal that cuts across party lines.
Cameron, who leads a two-party coalition, is under growing pressure from his own lawmakers over his stance on Britain's membership of the European Union and his decision to press ahead with gay marriage legislation.
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Both issues have divided the Conservatives, who trail the opposition Labour party by 10 percentage points in the polls and face a threat from a surging anti-European Union party, the UK Independence Party, which they have so far failed to stifle.
A small group of Conservative lawmakers even want to oust Cameron before the next election, but most deem that unlikely.
Johnson has long been tipped as a potential successor to Cameron and has made no secret of his ambition to succeed him if the position became vacant, while at the same time pledging loyalty to the prime minister.
"Grateful as I always am to Andy for career advice, I am backing David Cameron all the way," Johnson told reporters on Wednesday, saying he was increasingly confident Cameron would win the next election.
However, the intervention of Coulson, a former insider, is likely to resurrect speculation about the strength of Cameron's position.
"Boris Johnson desperately wants to be prime minister and David has known that fact longer than most," 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'." Johnson has been mayor since 2008.
Coulson advised Cameron, 46, to adopt a careful strategy.
"Support his good ideas, advise privately on the bad ones, but only engage publicly if absolutely necessary - and celebrate Boris's considerable successes," he wrote.
Coulson resigned from his job as Cameron's head of communications in 2011 after allegations he had known of phone hacking by journalists at the now defunct News of the World tabloid, of which he was previously editor.
Coulson, who denied wrongdoing, is due to stand trial over the matter later this year. He also faces charges related to allegations of bribing public officials.
(Reporting by Andrew Osborn; Editing by Mike Collett-White)
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