Behind the smiles and triumphant back-slapping that accompanied Boris Johnson's victory this weekend, David Cameron is privately uneasy that the new Tory Mayor of London could derail his chances of becoming Prime Minister.
Mr Johnson's performance as the most powerful elected Conservative in Britain since John Major's 1992 victory could decide the outcome of the next election and shape the course of politics for the coming decade.
And his pledge to water down the capital's congestion charge scheme and protect drivers of high-polluting cars threatens to clash with the central theme of Mr Cameron's blueprint for power: the environment. Other areas of contention include immigration.
The two men have a long relationship stretching back nearly 30 years, illustrated by the now notorious picture of them together as members of the Bullingdon Club at Oxford, and if they work together in a constructive way, party chiefs believe they will be an important axis in the Tories' return to power.
But there is distinct unease at what lies in store from the Johnson mayoralty. Friends say Mr Cameron regards his fellow Old Etonian as an "amusing liability" and a few months ago never believed he would win the race.
After a marathon 15-hour count, Mr Johnson beat the Labour mayor by 1,168,738 votes to 1,028,966 on a record turnout of 45 per cent. He promised his victory would not mean London had suddenly become a "Conservative city" and acknowledged his reputation will have caused some voters' pencils to "hover" over his name before they put a cross in the box.
But senior Tory strategists now face a daily battle to ensure that Mr Cameron's smooth path to No 10 is not suddenly obstructed by an unpredictable and gaffe-prone Mayor. If Mr Johnson is interested going further and making it to the Conservative Cabinet, he is likely to behave himself, they say. But he could decide to make his mark as an independent-minded Mayor who is not answerable to the Tory leadership.
The Cameron-Johnson relationship, which dates back to their membership of the Political Society at Eton, is described as "perfectly friendly". The Johnsons were at the Camerons' for dinner recently, while Samantha Cameron and Marina Johnson went on the campaign trail together.
Yet Mr Johnson has been passed over for preferment by Mr Cameron. He was appointed shadow education minister but never returned to the shadow cabinet after being sacked by Michael Howard for lying about his affair with the journalist Petronella Wyatt. One friend said the lack of other contenders for the Tory ticket last year had contributed to Mr Johnson becoming the "accidental Mayor". Another said last night: "No one really believed this would happen. He [Cameron] didn't exactly have an embarras de candidats last year."
One of those present when the candidates were told the result, just before it was announced publicly, said of Mr Johnson: "I have never seen anyone look so shell-shocked. He looked awful, a real 'Oh my God' look. He looked as if he'd really lost it. He really never expected to win."
On webcameron, the Tory leader's internet video diary, Mr Cameron was seen attempting high-fives with aides as the result was announced. His first comment was a restrained: "Exactly," before adding: "Brilliant."
In public, Mr Cameron said yesterday that Mr Johnson's victory shows that voters are ready to elect a Tory to high office because the party has changed. "Three years ago the idea that the Conservatives would win London and build up a 20 point lead across the county would have been literally unthinkable.
"London is one of the most diverse, vibrant, successful and important cities in the world – and in Boris Johnson it is now has a Conservative Mayor. We've shown there is an alternative. We must now prove it. But I believe that the Conservative Party – is ready to step up for Britain."
In a sign of how unpredictable the Johnson mayoralty could be, his sister Rachel told BBC News 24 on Friday that supporters had been drinking champagne and eating oysters on the top floor of Millbank, above Tory party headquarters, since 5pm – more than six hours before the result was announced.
And Mr Johnson's father Stanley said: "He's very good at Greek and Latin and I can tell you something – if you can do Greek and Latin you can do anything, certainly run a city like London."
Mr Johnson's victory was welcomed by a string of cabinet ministers – suggesting they expect him to be a loose cannon. He has signalled that he will defy the Tory leader if it suits him. He has pledged an amnesty for illegal immigrants – against official party policy – and is viewed as a climate change sceptic.
While the newly elected Mr Cameron was rebranding the party in 2006 – and starting to make it electable again – by unexpectedly turning it green, Mr Johnson repeatedly used his Daily Telegraph column to denounce environmentalists and their causes.
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