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Mr Carswell has quit as MP for Clacton, saying he will contest the subsequent by-election for Nigel Farage's party.

If he wins the support of voters he will be the first elected UK Independence Party MP in the Commons.

The maverick Eurosceptic backbencher said he wanted to "shake up" the cosy Westminster "clique".

But Montgomeryshire's Conservative MP Glyn Davies said he was surprised by the move.

Mr Davies initially tweeted his surprise at the shock announcement by saying he almost fell off a huge anaerobic digester he was looking at in south Shropshire when he heard the news.

The MP joked: "Was 100 ft up atop an anaerobic digester when heard Douglas gone over to UKIP.

"Wow, almost went over myself. The side of the anaerobic digester tower I mean."

Mr Davies later explained: "I was down by Craven Arms and up an anaerobic digester when I heard the news.

"I must say I am surprised but I am not going to be critical because I know Douglas, I respect him and I like him.

"But I must say I think what he has done is illogical and it is a bad mistake. In my view the Prime Minister David Cameron is the only politician who can deliver the opportunity for the British public to say whether they want to be in or out of Europe."

Wrekin MP Mark Pritchard said: "It comes as no real surprise, he has been flirting with UKIP for months.

"Trouble is that a vote for UKIP, in most seats, is likely to help Ed Miliband into Downing Street, who would deny the British people an EU referendum.

"Only a Conservative government will deliver a European referendum."

Shrewsbury MP Daniel Kawczynski said: "It is a regrettable and counter-productive decision.

The Conservative Party will contest the forthcoming by-election vigorously to ensure the people of Clacton have a strong Conservative voice in this Parliament and the next." Philip Dunne, MP for Ludlow, said: "I regret this move, since as Douglas himself has acknowledged, the only way to get a referendum on the EU is to return a majority Conservative Government.?"

But Jill Seymour, UKIP West Midlands MEP and transport spokesman who lives in Kynnersley, near Telford, welcomed Mr Carswell to the party and described his defection as "a definitive moment in modern UK democracy".

Mr Carswell said the decision to jump ship from the Conservatives had given him 'sleepless nights' but he wanted to see 'fundamental change in British politics' and UKIP – a party he believed belonged to its members rather than a 'little clique' of political insiders – could deliver it.