Shortly before Christmas, David Cameron phoned Boris Johnson at his home in North London. It was not a social call.

The Tory leader had been unsure from the start about Mr Johnson as a mayoral candidate. With three months to go before the start of the formal campaign there was little sign of progress. How, Mr Cameron inquired, did Boris intend to take the fight to Ken Livingstone?

The two men have known each other for almost 25 years. At Eton, at Oxford, in the media and in politics their lives have touched, diverged and touched again. Now their fates have became wound together in the most extraordinary fashion.

For the majority of their acquaintanceship, Mr Cameron has bobbed in the older man’s wake. Boris Johnson was enjoying fame even as David Cameron, then a 13-year-old new boy, first saw that blond mop. Honours fell to the young Boris almost in inverse proportion to the amount of effort he made to secure them. He was considered brilliant at Eton: Cameron was not for most of his school career.

Boris was elected to Pop, a pupil-selected elite: the schoolboy Cameron was not. It is not clear whether Johnson helped to elect his younger peer to the Bullingdon. It seems almost certain that he helped to trash Mr Cameron’s room at Brasenose College, the ritual welcome to Oxford’s most notorious dining club.

By the time that Mr Cameron joined Conservative Central Office his near-contemporary was already making his name as a journalist, most notably as The Daily Telegraph’sBrussels correspondent. It was said that Mr Johnson was Margaret Thatcher’s favourite journalist. Mr Cameron has admitted that his own encounters with Mrs Thatcher while she was Prime Minister were unlikely to have left her with a favourable impression, if indeed any impression at all.

The 1990s were a decade of brilliance for Boris, capped in 1999 with the editorship ofThe Specator.He was still only 35 but this prize was far from the limits of his ambition. As a youngster, he once confided to a friend that he would like to be President of the United States. (His birth in New York made this possible, albeit ludicrous).

By now Messrs Johnson and Cameron were joined by a third combatant, George Osborne. The three were asked by Iain Duncan Smith to help him to prepare for Prime Minister’s Questions. It was a demanding brief but one that Mr Cameron performed dutifully despite enduring a very difficult time after the birth of his disabled son, Ivan. While Mr Cameron arrived punctually at his party leader’s office at 7am, despite sometimes having spent the night on a hospital floor, Boris Johnson would breeze in hours later and contribute little of use.

For men such as Mr Cameron and Mr Osborne, veterans of the party machine, it would have been hard not to share in the whips’ view that their colleague lacked both seriousness and application. It was an opinion that Michael Howard barely bothered to conceal. When he took over as party leader he first left Mr Johnson out of his Shadow Cabinet and then sacked him from the front bench. Given that Mr Cameron was part of the Howard inner circle it seems likely that he had some part in this dismissal.

It must have stung deeply when Mr Cameron left him out of his first Shadow Cabinet on being made leader. Mr Johnson had been one of only 14 MPs to back him from the start in the 2005 leadership election. When mild criticism of Jamie Oliver by Mr Johnson provoked a media storm that threatened to overshadow Mr Cameron’s first speech as leader to a party conference in 2006, fears that Brand Boris was too big and dangerous increased.

Whatever he might say now, it is clear that Mr Cameron was not enthusiastic about a Boris candidature when it was first floated. For Mr Johnson it was a shot at redemption. “There is no doubt that there has been a growing-up process,” said an insider. “This was something where charm alone would never be enough.” By December Mr Johnson feared that his campaign was not equal to the task. “He started to worry, I think, that he couldn’t win it,” an observer said. Mr Cameron and, more particularly, Mr Osborne, who was handling the London mayoral campaign at Tory HQ, had come to the same conclusion.

The result was the arrival of Lynton Crosby. The Australian political consultant is credited with turning the campaign around. It was decided to pursue a strategy that concentrated on Tory-leaning boroughs.

Today Mr Cameron is full of praise, both in public and private, for his candidate.

The final credit for a Johnson victory would have to go to the candidate. He has laundered his reputation as a gaffe-prone, lightweight idler with a campaign of energy and discipline. Homer’sIliadis Johnson’s central text. Part of the charm of Boris Johnson is that he invites his public to see his life in heroic terms.

Even if his campaign were to end in defeat it would be cast as an heroic failure. But Boris thinks he is better than that. Flawed but brilliant, he wants to take his place in the field of honour where he has always thought that he belonged.

Ascent of Boris

— Born June 19, 1964

— School: Eton College (captain of school)

— College: Balliol College, Oxford University, 2:1 in classics

— Media CV: The Times (sacked) The Daily Telegraph, The Spectator

— Elected MP for Henley 2001, appointed to front bench 2004

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