David Cameron, the youthful Tory moderniser, will face off against David Davis for the leadership of the Conservative Party after scoring a clear - but not decisive - victory in the second ballot of MPs tonight.

Mr Cameron grabbed 90 votes, not quite half of the 198 cast. The real battle was for second place, where Mr Davis, the long-time frontrunner, managed to edge out Liam Fox, his rival on the Right of the party. The result was close, with Mr Davis picking up 57 votes, while Dr Fox received 51.

Mr Davis, the Shadow Home Secretary, earlier "definitely, absolutely" ruled out a suggestion that he could retire from the contest and allow Mr Cameron smoothly to be crowned leader to avoid a costly and potentially divisive head-to-head battle.

The two men now have six weeks to woo 300,000 party members around the country for a postal vote closing on December 5.

Although Mr Cameron's total vote far exceeded that of his rivals, it nevertheless fell short of some of the more optimistic forecasts from his supporters. It was clear, too, that many of those MPs who supported Kenneth Clarke in Tuesday's first ballot had given their backing to Mr Fox.

Mr Davis, however, will be concerned to see that his vote fell from 62 in the first ballot, in which Mr Cameron had scored 56 votes and Dr Fox 42.

As the decisive session of campaigning starts, Mr Cameron's hand was strengthened this morning by a newspaper poll suggesting that he enjoys an overwhelming lead among party members.

The YouGov poll in The Daily Telegraph, conducted online on Tuesday evening, suggested Mr Cameron had the backing of 59 per cent of party members against 18 per cent for Dr Fox and a meagre 15 per cent for Mr Davis.

Bookmakers have Mr Cameron as the virtually unbackable 12-1-on favourite to replace Michael Howard as the party leader, with Mr Davis the 6-1 outsider in the two-horse race.

And only minutes before voting ended, the Shadow Health Secretary received a major boost when Nadine Dorries - who seconded Mr Davis's nomination for the leadership - said she had decided to switch her allegiance to Mr Cameron instead, as the best man to win the next election.

Ms Dorries, the MP for Mid-Bedfordshire, said: "David Cameron’s campaign has been absolutely superb. If he runs a general election campaign like he has run his leadership campaign, we are home and dry. He makes Tony Blair look old, tired and shallow."

The leadership contest will not just pit the Tory Left against the Tory Right - although those labels have lost much of their meaning. It will also pit an Old Etonian born with a silver spoon in his mouth against a man brought up on a Tooting council estate who went on to become a part-time SAS commando and successful businessman.

Indeed the head-to-head campaigning began within minutes, as the two men left the Commons committee room where the result was announced, to fire the first of many barbs at each other.

Mr Cameron, clearly stressing his relative youth, said he wanted to be a "voice for change, for optimism and for hope". He will launch his campaign tomorrow with his first visits to constituency parties and an internet webcast.

He said: "I want a party that looks to the future, a party that is a 21st century party and is modern and compassionate and understands the aspirations and hopes and dreams of the people."

Mr Davis, for his part, appeared to be playing up what is seen as his rival's big weakness: that as a public schoolboy who went straight into politics after Oxford, Mr Cameron is out of touch with large parts of the country, and lacks the deep-rooted beliefs to become prime minister.

"There’s a long time to go in this contest yet, still another six weeks. I intend to fight for my beliefs," Mr Davis said.

"I’m going to start the process by going to my old University of Warwick tomorrow to carry to members of the party and others my beliefs - an opportunity society, in hope for Britain, in changing Britain to improve lives, and particularly to address the issue of reaching those parts of Britain that the Conservative Party hasn’t reached for too long."

Mr Cameron’s campaign manager, George Osborne, said tonight’s result was "tremendous" for his candidate. "It’s a tremendous result, exactly in the range we were predicting," he told Sky News. "David Davis has dropped some votes. This is a contest we are relishing."

Mr Osborne denied that Mr Cameron’s privileged background would prevent him from widening the Tories’ appeal, particularly in the North. "It’s not where you’re from, it’s where you’re going," he said. "When you look at the youthful appeal of David Cameron, that generational appeal - that’s exactly what we need if we are to win back all those Labour seats that we have lost."

Dr Fox showed no sign of disappointment as he emerged from the Commons smiling, accompanied by his fiancee, Jesme Baird. He congratulated his rivals and said either would make a great prime minister but refused to say immediately which of them he was going to endorse.

"I want to endorse the agenda that I have set out in the last few weeks," he said. "I want to see a candidate who will stand up for Britain's interests in Europe and an ever-changing world, and ensure a compassionate Britain at home. I will be listening with great interest to what the candidates say over the coming weeks."

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