London (AFP) - British Prime Minister David Cameron suffered a heavy blow on Thursday when one of his lawmakers, Douglas Carswell, announced he was leaving the Conservative Party to join the eurosceptic UK Independence Party (UKIP).
Carswell is to resign his seat and stand as a UKIP candidate at an upcoming by-election which, if he wins, would make him their first elected member of the House of Commons.
Polls suggest UKIP could take a handful of seats and thousands of votes from the Conservatives at the 2015 general election, which may increase the chances of victory for the opposition Labour party and defeat for Cameron.
Experts say Clacton in the southeast of England is among the Conservative seats most vulnerable to being captured by the eurosceptics at the election.
Cameron called the move "deeply regrettable" and "self-defeating" and vowed the Conservatives would put up a "very strong" fight in the by-election.
But Carswell accused him of misleading voters over plans to claw back more powers from Brussels ahead of a referendum on Britain's membership of the European Union, which will take place in 2017 if the Conservatives win next year.
"My position became untenable," said Carswell. "The Conservative leadership is not serious about change."
He added that Cameron's advisers had told him Britain will not vote to leave the EU in the event of a referendum, because "we will give them (the voters) just enough to persuade them not to".
The prime minister promised the referendum under pressure from the powerful eurosceptic wing of his party and to win back voters from UKIP, which won May's European Parliament elections, beating the Tories into third place.
That was the first British poll victory for a political grouping other than the Conservatives and Labour for more than a century.
"We have had a duopoly for many decades," said Carswell. "We need choice and competition in politics."
- 'Bolt from blue' -
Carswell also slammed Cameron's decision to rule out an electoral pact with UKIP, pointing out that the Conservatives had been less reluctant to form a coalition government with the centre-left Liberal Democrats following the 2010 general election which remains in power.
Cameron hit back, telling the BBC that it was "deeply regrettable when people behave in this way, and also counterproductive."
"I'll want to make sure there is a very strong Conservative campaign," he said of the by-election.
UKIP leader Nigel Farage, who appeared at his new recruit's side at a press conference to announce the resignation, said it was "the noblest thing I've seen in British politics in my lifetime".
Carswell has earned a reputation as a staunch eurosceptic and radical libertarian since he was first elected in 2005 in nearby Harwich.
He was at the heart of a succesful bid to remove then speaker of the House of Commons Michael Martin following an expenses scandal which rocked parliament in 2009.
The by-election's date has yet to be announced, but Carswell is considered to be in a very strong position.
Matthew Goodwin, an academic and leading authority on UKIP, wrote on Twitter: "Clacton is THE most favourable seat for UKIP. Carswell will win hands down."
The news came as a "bolt from the blue" to his constituency.
"We always thought he was a Conservative, very much against Europe, but we thought he would work within the party to achieve his aims," said Mick Page, the Conservative leader of the council which covers Clacton.
Farage announced on Tuesday he was to run for parliament in next year's election after his party confirmed him as their candidate for another coastal constituency, South Thanet.