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 issued a "wake-up call", warning that Labour could pay a heavy political price if it continues to dismiss the Mayor of London as a lightweight.
One senior Labour source told The Independent: "Boris Johnson 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. A lot of things that appear to be spontaneous are deliberate. 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. "We take the view that we are the only party who can be really confident that its current leader [Ed Miliband] will still be leader at the election," said the Labour source.
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.
Sunder Katwala, former head of the Labour-affiliated Fabian Society who now runs the British Future think tank, will say: "Labour cast him as a clown and could not believe the voters would ever elect him, right up to the moment that the first did. By helping to set expectations so low, implying that Boris could never be up to the job and almost that it would be a miracle if London survived his mayoralty, this Labour narrative massively boosted his chances of re-election.
"If Labour doesn't change its tune, it could end up letting him walk into Downing Street."
Meanwhile 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.
From gay marriage to the EU (and zip wires): Johnson's populist appeal
From the day when he was elected Mayor of London, 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.
Taxing the rich
He campaigned long and unsuccessfully for the 50p top tax rate to be scrapped. The Government has dared to reduce it to 45p, and for the Tory faithful, Boris got the credit for fighting the fight – even though he was only partially successful.
On another issue dear to Tory hearts, Johnson said in October 2011 that a referendum on membership of the EU would be "not a bad idea" if a "reasonable" question could be framed. David Cameron is opposed to holding an 'in-out' referendum, much to the anger of many Eurosceptics in the party.
Two years ago, Boris Johnson wowed the Conservative conference with a call for tighter anti-strike legislation. He suggested that a strike should be legal only if half of the total union membership voted for it, as opposed to a simple majority of those who turned up to vote. He repeated that call six months later, saying that the Government was being "lily-livered" in its handling of the unions.
Boris has managed to expound right wing views that appeal to hard line Tories without sounding like a backwoodsman. This is partly because of his social liberalism. In August, he recorded a YouTube video arguing the case for gay marriage.
There is also the panache with which he handles events like the London Olympics. His speech at the closing ceremony drew louder applause than Mr Cameron's, and he even milked the situation when he got stuck on a zip wire. As David Cameron said: "If any other politician anywhere in the world was stuck on a zip-wire it would be a disaster. For Boris, it's an absolute triumph."
Spectre of Bojo follows the PM around the globe
Even on the other side of the world, Prime Minister David Cameron was unable to escape Boris, as journalists quizzed him about a leadership challenge yesterday – in Brazil.
Folha de São Paulo asked: "We see mayor Boris Johnson with greater popularity to the point there are rumours he might be the Conservative candidate for the next general election. Do you feel your post is threatened?"
Mr Cameron said: "Boris still has much work to do as mayor, and so do I as Prime Minister."