British Prime Minister David Cameron faces a potentially damaging by-election after Tory MP Douglas Carswell dramatically defected to Ukip.

In a bombshell press conference yesterday, the backbencher lambasted the UK leader 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,” 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?”

Carswell said he had been an enthusiastic cheerleader for 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.

It’s above all the failure to deliver on the promise of political reform that has driven me to be here

“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.”

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.

Carswell said he would be resigning from Parliament as from yesterday. He was originally elected as MP for Harwich in 2005, but was returned in Clacton in 2010 after boundary changes with a majority of more than 12,000.

The Tories could be in for a major battle to retain the seat, as 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.”

Leader of the House of Commons William Hague said: “It is a regrettable and deeply counterproductive thing to do, because the only chance of real change in Europe and upholding a referendum in this country in which the people of this country can decide to stay in or leave the EU is the election of a Conservative government next May, a majority Conservative government.

“Anything that makes that harder is damaging the chances of real change in Europe.”

He pointed out that Carswell had himself stated that on Twitter in March.

“That is as true today as it was a few months ago, when he said it. We have already shown we can achieve change in Europe,” he said.

Ukip leader Nigel Farage welcomed Carswell to the party and described him as “just about the most perfect fit”.

He also commended his actions as the “bravest, most honourable and noblest” he had witnessed in British politics during his lifetime.

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