He joined the Conservatives 40 years ago and fought a number of elections for the party he loved, taking on Tony Blair in Sedgefield in 1983. He was constituency chairman to William Hague, when he was in the Cabinet and the party’s leader.

But the Tory party’s recent embrace of the left-wing columnist Polly Toynbee has stretched Toby Horton’s loyalty beyond breaking point.

In the most high-profile of a wave of defections that is shaking the party leadership, Mr Hague’s former constituency chairman has resigned from the Conservative Party and joined the United Kingdom Independence Party. Other defections include a local party chairman and another former parliamentary candidate.

Conservative MPs said that these were warning shots across the bows of David Cameron’s campaign to take the party to the centre ground, and declared that many more defections were likely. UKIP is hailing the defections as proof that it is now the true party of the Centre Right.

“I didn’t leave the Conservative Party, the Conservative Party has left me,” Mr Horton said yesterday at his home in Mr Blair’s constituency. “There is a real need in this country for a party of the Centre Right, and if the Conservative Party doesn’t want to fill it, there is inevitably a vacuum that UKIP will fill. If you want to vote green, there is a thriving Green Party. If you want to vote Liberal Democrat, there is a thriving Liberal Democrat Party.”

Mr Horton’s frustration has been building as Mr Cameron has tried to shed the party’s “nasty” image by ditching its past, focusing on issues such as the environment and social justice, and promising to show love to louts. But the final straw for Mr Horton came last month when the party embraced Ms Toynbee, who has made a career out of attacking all things right-wing. A Conservative policy review suggested that the party adopt her views on poverty rather than Winston Churchill’s, and Mr Cameron suggested that she would be invited to next year’s party conference.

“There was a Polly Toynbee moment — that was the tipping point,” said Mr Horton, who met Mr Cameron at the hustings during his leadership campaign.

He gave warning that many people would follow him. “It will become increasingly common. Most Conservatives are instinctively loyal, but there’s always a tipping point.”

The defection of such long-serving and high-profile Conservatives has caused alarm among the party’s MPs. One said: “This is undoubtedly a warning shot. Many people tempted to join UKIP are cast-iron Conservatives, but they must feel they have something to vote for. It is essential we keep on board traditional Tories.”

Another defector is Mark Hudson, 36, a vice-president of the Bank of America, who was president and chairman of the Sevenoaks Conservative Association in Kent, and on the official Conservative candidates’ list.

“The way Cameron has led the party means I can’t be part of it, and there’s lots of people like me,” he said. “He’s turned it into something indistinguishable from new Labour, so what’s the point?” Kevin Davidson Hall, a former Conservative candidate in Peterborough, has also defected, declaring: “UKIP are now the only voice of opposition in this country, speaking up for all those who feel betrayed by the other parties.”

Nigel Farage, the UKIP leader, said: “These activists know it makes no difference who wins the election — their manifestos are pretty much identical.

They’re pretty cheesed off. We are the only ones offering real commonsense policies.”

The defections are the latest turbulence to hit Mr Cameron’s changes to the party. On Tuesday Paul Offer, the selected candidate for Chester, resigned in protest at the racist and sexist attitudes of local party officials, prompting a formal investigation.

Labour said that it showed how deep the problems were in the Conservative Party. Denis MacShane, a former Europe Minister, said: “This shows the failure of Cameron to get any grip on his party.”

A clearly rattled Conservative headquarters issued a statement declaring: “UKIP is a party of the few dedicated to a single issue. The Conservative Party is the only political party that can replace this failing Labour Government.”

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