LONDON (AFP) - British Prime Minister David Cameron suffered a heavy blow on Thursday when one of his lawmakers, Mr Douglas Carswell, announced he was leaving the Conservative Party to join the eurosceptic UK Independence Party (UKIP).

Mr 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 be poised to take a string of seats from the Conservatives at the 2015 General Election, raising the chances of a victory for the opposition Labour party and defeat for Mr Cameron.

Experts say Clacton, up the south-east coast from the seat where UKIP leader Nigel Farage will stand in 2015, is among the Conservative seats most vulnerable to being captured by the eurosceptics at the election.

Mr Carswell accused Cameron 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 Mr Carswell. "The Conservative leadership is not serious about change."

He added that Mr 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," explained Mr Carswell. "Look at how the country has been run. It has been a competition for cliques to sit on the sofa.

"We need choice and competition in politics."

Mr Carswell also slammed Mr Cameron's decision to rule out an election pact with UKIP, pointing out that the Conservatives had been less reluctant to form a government with the centre-left Liberal Democrats following the 2010 General Election.

The outgoing MP said it was right to hold a by-election, even though he could have seen out his term as a UKIP representative.

"As someone who's always answered directly to the independent-minded people of Essex, there is only one honourable thing for me to do," he said.

Mr Farage, who appeared at his new recruit's side at a press conference to announce his resignation, called Mr Carswell's decision "the noblest thing I've seen in British politics in my lifetime".

Mr 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 the then speaker of the House of Commons, Mr Michael Martin, following the 2009 expenses scandal that rocked Parliament.

Some experts predicted he would have little trouble securing re-election at a by-election whose date has yet to be set.

Mr 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 Mr Mick Page, the Conservative leader of the council which covers Clacton.

"As far as I'm aware the Conservative association had no warning of this."

Mr Farage announced on Tuesday he was to run for Parliament in next year's election after his party confirmed him as their candidate for South Thanet.