David Cameron has described Boris Johnson's re-election for a second term as London Mayor as his "number one priority" for 2012.
In a speech last night to Conservative backbenchers, the Prime Minister described victory in the capital city as "essential" to the party's strategy.
But he warned that next year would be "tougher than the first couple of years under Margaret Thatcher" because of the state of the economy.Mr Cameron was given thunderous applause at the mention of the London election battle on May 3.
According to an insider, he said there were other English and Scottish local elections on the same day but "keeping Boris in London was absolute priority".
Conservative deputy chairman Michael Fallon said: "Boris's election is the Big One. The whole party will be working flat out to make sure he wins London again."
The most recent London opinion poll put Mr Johnson ahead of Ken Livingstone by eight points, with 54 per cent of the vote. But Labour are confident that Londoners have swung to them since the Coalition was formed last year and that it will be a close race.
The importance to Mr Cameron, say senior Tory MPs, is that his policies and the economic slowdown would be blamed if Mr Johnson lost.
At a private dinner this year, the PM is said to have told the Mayor: "Just in case it needs saying, you know that not only do I want you to win next year because we are friends, but I recognise that there is no way you losing would be seen as anything but a disaster for me."
London has done well in the Treasury's spending reviews, with projects such as the £16 billion Crossrail scheme being secured.
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However, Mr Johnson and Mr Cameron have clashed over policy several times. The Mayor has attacked benefits cuts for risking "Kosovo-style ethnic cleansing", campaigned for the scrapping of the 50p rate tax cuts and backed a referendum on the EU.
