Yeo called on other centre-left candidates to move aside to give the party’s best-known “big beast” a clear run against the right-wing David Davis, still the favourite to replace Michael Howard.

After a lacklustre start to his leadership bid, Clarke is gaining momentum. Last week, his sceptical remarks about the euro and the European Union constitution delighted Tory MPs who had felt that his long-held Euro-enthusiasm would scupper his chances.

In an interview in today’s Sunday Times News Review, Clarke argues that he is the candidate most feared by Labour and the Liberal Democrats.

“Elections are fought in the centre,” he says. “My strongest appeal is that I have higher approval ratings than any other contender, higher still among wavering Labour and Liberal Democrat voters. The Lib Dems are terrified of me.”

He attacks the way the party has lost support among “the educated, professional, managerial and women”. He says: “I find it almost comic the number of people who tell me they would vote Tory if I were leader.”

Yeo, MP for South Suffolk, resigned from the shadow cabinet after the last election. He had little prospect of victory but is a respected figure on the centre-left of the party. He will take to the airwaves this morning to drum up support for Clarke. His decision to endorse his former rival will be a blow to the faltering campaign of David Cameron, the other “one nation Tory” hopeful who may have expected to win Yeo’s backing.

Announcing his withdrawal from the race to The Sunday Times, Yeo said: “The most important consideration for the party is who can lead us to victory. Unquestionably the most likely person to defeat Labour and to see off the threat from the Liberal Democrats is Ken Clarke. I hope my supporters and colleagues, including those still thinking of running for the leadership, will follow my lead and back a winner.”

Davis remains the favourite although the odds are now on a run-off in November with Clarke. Davis, the shadow home secretary, accuses his rival of “newspaper manoeuvring” and says the recent statements retreating from his pro-European stance would not make him a more dangerous opponent.

Privately, Davis’s supporters say that Clarke — who stood unsuccessfully for the leadership in 1997 and 2001 — is too old and that his credibility as leader would be damaged by his europhile history and his close association with the defeated and demoralised government of Sir John Major.

Senior figures led by Lord Heseltine, the former deputy prime minister, are urging Cameron to run as Clarke’s deputy on a “dream ticket”. Cameron, however, is being urged by his supporters to stand firm and offer a young, fresh alternative to the other two.

Yeo’s intervention will intensify pressure not only on Cameron but fringe candidates such as Malcolm Rifkind and David Willetts, who are both seen as centrist contenders. Willetts has indicated he might stand aside if another candidate ran on an agenda that was close to his own socially inclusive ideas.

Clarke hopes to hold talks with him this week to persuade Willetts that their approaches are almost identical.

But Cameron does not seem willing to drop out of the race just yet. He said of Clarke in an interview today: “He has got a lot to offer the party. But I don’t believe that the right way to go about this is talking about deals.”

“Ken is the best qualified and most popular of all the potential candidates,” said Yeo. “Crucially he has the strongest appeal to people who do not currently vote Conservative — the very support we must regain if we are to win the next election.

“Ken is supremely placed to take the argument to Gordon Brown and win; he has done it before and will do it again.

“Put simply, Ken is the man to lead the Conservative party into government once again.”

Clarke indicated it would be an “honour” to lead the party after being accused of lofty indifference in his two earlier bids for the crown. In an appeal to the party’s thirst for power he indicated he would even consider a coalition with the Liberal Democrats and asked his colleagues: “How much do [you] want those red boxes?”

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