Conservative MP Douglas Carswell has defected to the UK Independence Party and will stand down as MP for Clacton in order to trigger a by-election.
He will stand for re-election as a Ukip candidate and, if he wins, will become the party's first MP.
Carswell is renowned for his anti-EU stance and has been a frequent rebel on European issues.
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His resignation, nine months before the general election, comes as "a major blow" to David Cameron, the Daily Mail says.
Carswell said that he was resigning his seat in Parliament because the prime minister was not "serious about real change".
He said the decision to abandon the Tories had caused him "sleepless nights" but he wanted to see "fundamental change in British politics" and felt that Ukip could deliver it, the BBC reports.
"It's above all the failure to deliver on the promise of political reform that has driven me to be here today," Carswell said. "Europe's the one continent on the globe that is not growing... Yet who in Westminster, who among our so-called leaders, is prepared to envisage real change?"
Carswell won his first election in 2005 by a margin of just 920 votes, but was returned in 2010 with a 12,000-vote majority.
Ukip leader Nigel Farage said Carswell's decision was the "bravest and most honourable" move he had ever seen in British politics.
But in The Times, Philip Webster said that growing support for Ukip on the back of a by-election victory could have unintended consequences for those who want to see Britain leaving the EU.
The Conservatives stand to lose most from a surge in anti-European sentiment, he said, and "if Ukip's progress does hand the election to Ed Miliband it will not get the referendum it wanted on UK membership of the EU".
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