Gordon Brown has all but written off Ken Livingstone’s chances of winning the London mayoral election, according to close allies.

Senior figures in Downing Street believe that if Mr Livingstone wins it will be by the narrowest of margins.

Mr Brown revealed yesterday that he plans a trip to the US as the fight for London reaches a climax next month. David Cameron has chosen to appear alongside Boris Johnson when the Conservative candidate officially begins his campaign next week.

Some Brown allies are already consoling themselves that a Johnson victory “would be a disaster” for Mr Cameron. Others, including senior Cabinet supporters, fear that a Johnson mayoralty would hand the Tories a prize platform for building bridges with the business community in the City.

Senior staff on the Johnson campaign say that Mr Cameron’s explicit endorsement reflects growing confidence in Conservative headquarters about his chances of deposing Mr Livingstone on May 1.

Mr Johnson is benefiting from poll leads for the Conservatives nationally in recent weeks, with the most recent survey suggesting that he enjoys a 12-point advantage over the incumbent.

His campaign will intensify next week, say insiders. The party’s leadership made the appointment of Lynton Crosby, the Australian strategist, a condition of continuing campaign finance.

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