The Conservative leadership candidate Sir Malcolm Rifkind has withdrawn from the contest to succeed Michael Howard and given his backing to Ken Clarke.
The decision by the Kensington and Chelsea MP had been expected after Sir Malcolm, a former defence and foreign secretary, failed to gather more than a handful of supporters among Conservative colleagues.
Only six MPs publicly supported him and he was trailing badly in polls of Conservative activists and voters.
His departure leaves four men in the race to succeed Michael Howard: the bookies' favourite, David Cameron, the former chancellor Kenneth Clarke, the shadow home secretary, David Davis, and the shadow foreign secretary, Liam Fox.
Sir Malcolm last week told Conservatives at the party's annual conference that they could only win the next election if they occupied the centre ground of politics.
Before his conference speech, Sir Malcolm was rated by bookmakers as a 50-to-1 outsider for the leadership role, and he admitted he had a "mountain to climb" to win the necessary backing.
In a letter to party activists in Reigate and Banstead, his campaign manager, Crispin Blunt, said Sir Malcolm had withdrawn his name from the ballot of MPs due to take place on October 18 because he had realised there was "insufficient support to give him any realistic prospect of success".
Mr Blunt later told reporters Sir Malcolm's withdrawal was "the responsible thing to do".
He said Sir Malcolm had "not been able to get over the handicap of not being in parliament for the past eight years", which had hindered his chances of convincing two new intakes of Tory MPs to back him.
Mr Blunt called Sir Malcolm a "fantastic parliamentary peformer", but admitted that the entry of Ken Clarke into the race had made it an "uphill battle" to secure the mantle of one nation Toryism. He confirmed that Sir Malcolm would be backing Mr Clarke.
Sir Malcolm himself, speaking at lunchtime, admitted he couldn't "deliver a bloc" of votes to Mr Clarke, but said the former chancellor was "head and shoulders" above the other candidates.
"He is a potential prime minister and we are not just choosing a leader of the opposition," he said.
The shadow work and pensions secretary, Sir Malcolm, 59, was a government minister in the Thatcher and Major administrations, joining the cabinet in 1986 as Scottish secretary and rising to be foreign secretary from 1995 to 1997.
He lost his Edinburgh Pentlands seat in the 1997 Labour landslide and spent eight years in the political wilderness before returning to Westminster as MP for Kensington and Chelsea in May this year.
It is thought his return to parliament was largely motivated by his desire to lead his party, and he was the first to declare his intention to stand for leadership after Mr Howard said he was planning to step down.
His withdrawal from the race comes after a poll this morning gave him just 2% of support among Tory voters.
The Populus survey, for the Times, put Mr Cameron and Mr Clarke level, on 33%, with Mr Davis on 12% and Dr Fox on 7%.
Later, the Conservatives' deputy leader and shadow defence secretary Michael Ancram - who had been backing Sir Malcolm - declared his support for Mr Cameron.
Mr Ancram, who at one point was tipped to stand for the leadership himself, said in a statement: "I have decided to back David Cameron because having watched him over these last few weeks I have come firmly to the conclusion that he is best placed to lead the Conservative party to victory at the next election.
Mr Cameron's team said that 29 MPs have now backed the shadow education secretary publicly.
