Senior Labour figures are warning the party to stop treating Boris Johnson as "a joke" because they might be aiding his attempt to become Prime Minister – and defeat Labour at the next general election.
They have warned Labour could pay a heavy price if it dismisses the Mayor of London as a lightweight.
One senior Labour source said: "Boris has got where he is today by people not taking him seriously and saying he is not a threat. He thinks very carefully about his next move. There is method in his madness."
Labour's rethink follows speculation among Tory MPs that Mr Johnson could return to Westminster in a by-election and challenge Mr Cameron for the Tory leadership before the 2015 election – and reports that Nick Clegg could be forced to stand down as Liberal Democrat leader by then.
A week before he takes centre stage at the Tory conference in Birmingham, the "Boris effect" will be discussed in the margins of the Labour conference in Manchester, which starts this weekend.
A fringe meeting on Sunday will be told that Labour played into Mr Johnson's hands and helped him defeat Ken Livingstone to win a second term at City Hall in May.
Douglas Alexander, the shadow Foreign Secretary, told the New Statesman magazine: "I think it's time that we take Boris seriously… It is not yet a probability but it is a possibility that [Boris] will lead the Conservative Party into the next general election. He's managed to put a smile on voters' faces quite regularly. People feel he doesn't play by the rules."
Mr Johnson will also address a fringe meeting in Birmingham and some Tory MPs fear he will eclipse Mr Cameron, as he did during the London Games.
Boris Johnson has used the limelight to carve out a distinct political position – socially liberal, yet to the right of David Cameron on economic issues. He campaigned relentlessly for the 50p top tax rate to be scrapped, has described a referendum on membership of the EU as "not a bad idea" and once called for tighter anti-strike legislation. But he also recorded a YouTube video arguing the case for gay marriage.
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