Clacton MP Douglas Carswell yesterday left the Conservative Party and leapt into the arms of Nigel Farage’s United Kingdom Independence Party.

Mr Carswell has been Conservative MP for the Essex constituency – which also includes Frinton, Walton and Jaywick – since 2005 but has always been seen as a “free-thinker” by his former party’s hierarchy.

Some of his less charitable Tory colleagues always described him as a “loose cannon.”

Prime Minister David Cameron described his action as “self-defeating” and promised a strong Conservative campaign to hold it.

Mr Cameron said: “It is obviously deeply regrettable when these things happen and people behave in this way. But it is also, in my view, counter-productive,” he said.

“If you want a referendum on Britain’s future in the EU, whether we should stay or go, the only way to to get that is to have a Conservative government after the next election. That is what until very recently Douglas Carswell himself was saying.”

Mr Carswell’s defection to UKIP was announced by Mr Farage in a hastily-arranged press conference which had more than a touch of the cloak and dagger about it – UKIP’s regional staff in East Anglia knew nothing about it.

The surprise was doubled when Mr Carswell announced that not only was he defecting, he was also resigning from parliament to cause a by-election – which is likely to be held only six months before next year’s general election.

It also came as a surprise to county councillor Roger Lord who had been selected to fight the Clacton seat at the next general election.

Mr Lord told BBC Essex: “It is an enormous discourtesy, for anybody really, just to announce that and say that.

“I think the last time I saw him, I said I would be prepared to give him a reference come next May if he found himself unemployed, which I am pretty sure he would be.

“Perhaps he is jumping ship to try and get in front of all the other Conservative MPs who are going to find themselves in the unemployment queue come next May.”

The Ukip spokeswoman said: “Roger Lord has never been the by-election candidate for Clacton.

“By-election rules determine that the party is entitled to start with a clean slate of nominations for Prospective Parliamentary Candidates, with the final elected candidate being decided by the National Executive Committee.

“Roger Lord is a committed member of Ukip and we appreciate his contribution to the development of the party locally and hope that he understands the decision of the NEC to support Douglas Carswell in the defence of his seat, as a Ukip candidate.”

Mr Carswell said he had written to every member of his Conservative association to let him know about his decision.

“It is only right and proper I stop sitting in the House of Commons and I resign as an MP. I will do that formally today.

“I think it is right and proper I put myself forward as the UKIP candidate and I trust the people. It is up to them to decide if this is the right choice. I am seeking their permission.

“Throughout the years there have been all sorts of MPs who have moved from one party to another, but they have always found an excuse not to actually open themselves to a by-election.

“That would be dishonourable. The only honourable thing for me to do is to say the people I represent in parliament – ‘It is your choice’.

Mr Carswell said: “Don’t get me wrong, I have a lot of time for David Cameron. I like him as a person. I am not expecting him to send me Christmas cards anymore, but he is a decent person.

“I just don’t think he is serious. He is not serious about the change this country needs. He understands on one level the sort of political reforms we need.

“It was all there in the Conservative Party manifesto. It was all there in the coalition agreement, but he has not made it happen.”

Mr Carswell’s Conservative neighbour, North Essex MP Bernard Jenkin warned that his defection could boost Labour if Tory voters followed him in switching to UKIP.

And Colchester LibDem Sir Bob Russell was very blunt in his condemnation of Mr Carswell: “He’s so far to the right of the Conservative Party, I’ve wondered for some time how he could remain a Conservative!”

Mr Carswell took the Harwich seat off Labour incumbent Ivan Henderson in the 2005 general election, but in 2010 the boundaries were significantly changed and the strong Labour areas of Harwich were moved into the neighbouring North Essex seat.

The result was that both of the seats became relatively safe for the Conservatives, and Mr Carswell converted his 920 majority into a majority of more than 12,000.

In 2010 he was one of few Conservative MPs who did not face a UKIP challenge because the anti-Europe party felt he supported their aims.

In his own Conservative literature, Mr Carswell has published an endorsement from the UKIP leader.