David Cameron faces a potentially-damaging by-election after Tory MP Douglas Carswell dramatically defected to Ukip.
In a bombshell press conference, the backbencher lambasted the Prime Minister and senior Conservatives for not being “serious about real change”.
Among a list of criticisms, he insisted that the failure to take a stand against the European Union was at the heart of his decision.
“They are not serious about real change. It’s above all the failure to deliver on the promise of political reform that has driven me to be here today,” he 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?”
Mr Carswell said he had been an enthusiastic cheerleader for Mr Cameron’s Bloomberg speech, in which he set out proposals for an in-out referendum by 2017 if the Tories are in power after next year’s general election.
But he said he did not believe the policy was “sincere”, saying the leadership wanted to secure “just enough” to pretend change was happening.
“No one cheered DC’s Bloomberg speech more loudly than me... but there’s been nothing since. They haven’t thought it through.
“There is a world of change and opportunity out there... ministers are simply not up to giving us the kind of realignment we need.”
Mr Carswell said it would have been easier for him to “muddle along” as a Tory backbencher until next May, but he wanted to do the “honourable” thing.
“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.
“I must seek permission from my boss, the people of Clacton. I will now resign from Parliament and stand for Ukip in the by-election that now follows.”
The Clacton MP entered Parliament in 2005, and has frequently rebelled over European issues.
Ukip’s only representation in the Commons to date came when Castle Point MP Bob Spink defected from the Conservatives in 2008, although he subsequently stood unsuccessfully as an independent at the 2010 general election.
Mr Carswell said he would be resigning from Parliament today. He was originally elected as MP for Harwich in 2005, but was returned in Clacton in 2010 after boundary changes with a majority of over 12,000.
The Tories could be in for a major battle to retain the seat, as Mr Carswell is known as an independent-minded politician and is believed to have a significant personal following.
Following his announcement at the press conference, Mr Carswell said he wanted to see “fundamental change” in British politics.
He went on: “We have had a duopoly for many decades. 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.”
He said part of the appeal of Ukip was the fact it was a mass membership organisation that was the “property” of its members, adding that if he believed the Tory leadership was serious about change he would not have defected.
But he insisted the decision had not been an easy one, adding: “I have been a member of the Conservative Party for all my adult life. It is full of wonderful people who want the best for Britain.
“The problem is that many of those at the top of the Conservative Party are simply not our side. They aren’t serious about the change that Britain so desperately needs.
“Of course they talk the talk before elections. They say what they feel they must say to get our support when they want our support.
“But on so many issues, on modernising our politics, on recall of MPs, on controlling our borders...on bank reform, on cutting public debt, on an EU referendum, they never actually make it happen.”
He continued: “All three of the other parties seem the same. They have got swathes of safe seats, they are run by those who became MPs by working in the offices of MPs.
“They use pollsters to tell us what to tell the voters. Politics to them is about politicians like them. It is a game, a game of spin, position. First under Tony Blair, then under Gordon Brown, now David Cameron.
“It is all about the priorities of which ever tiny clique happens to be sitting on the sofa in Downing Street, different cliques, same sofa.
“Few are animated by principle or by passion. Those that are soon get shuffled out the way.”
A Conservative spokesman said: “This is a regrettable and frankly counterproductive decision. As Douglas Carswell said, the only way to get a referendum on the EU is to return a majority Conservative government.”
Mr Carswell said only Ukip could “shake up that cosy little clique called Westminster”, adding: “Many are just in it for themselves. They seek every great office, yet they believe in so little. Only Ukip can change this.”
He also dismissed the idea that the party was an “angry backlash against the modern world”.
He insisted Britain was a much better place than when he was born in the early 1970s except when it came to its political system.
“We are more open and tolerant. We are for the most part more prosperous,” he said. “More people are free to go out and lead the lives they want to lead than ever before.
“What was once dismissed as political correctness gone mad, we now recognise as just straightforward good manners.
“So much about Britain is so much better, except when it comes to how we do politics.
“They (the leaders) don’t think things through, they make one glib announcement after another.Then they move on to the next speech, the next headline.
“People have the right to expect a Government that is answerable to Parliament and a Parliament that is meaningfully accountable to the people.”
He stressed he was not against immigration but said it was “reasonable” to be able to decide who comes to the country.
“Just like Australia and Switzerland, we must welcome those who want to come here to contribute. We need those skills and drive,” he said.
“There is hardly a hospital, a GP surgery, a London bus, a supermarket that would run without that skill and drive.
“We should speak with pride and real respect about first-generation Britons. But, like Australia, surely it is reasonable that we should be able to decide who comes?”