David Cameron has conceded that his efforts to renegotiate the UK's relationship with Brussels would fail to appease hardline Eurosceptics within his own party who are determined to leave the European Union no matter what changes he could achieve.
The Prime Minister's assessment of the difficulties he faces with some in his own party came as Douglas Carswell formally quit as an MP after his shock defection from the Tories to Ukip.
Mr Cameron's difficulties over the European issue intensified as Ukip donor Stuart Wheeler confirmed he wined and dined around eight backbench Tory MPs to encourage them to switch allegiance.
The Prime Minister was forced to answer questions about Mr Carswell's defection during a Downing Street press conference focused on the terror threat facing the UK.
He insisted his plan for renegotiation followed by an in/out referendum by the end of 2017 was the best approach and repeated his assertion that Mr Carswell's exit from the party was "bizarre" given the promised public vote on EU membership.
Mr Cameron said: "Of course there are those in the Conservative Party, and people who vote for the Conservative Party, as there are people who vote for the Labour Party, who want to leave the EU altogether.
"Some of them want to leave the EU altogether irrespective of any renegotiation that I manage to complete.
"Indeed, before his defection I was pretty confident that Douglas Carswell was one of those people."
Mr Cameron added: " That is why I think his decision is slightly bizarre because he fought as a Conservative in 2010 when we didn't have a commitment to an in/out referendum but he has left the Conservative Party at a time when we do have a commitment to an in/out referendum.
"That is a question for him to explain, perhaps, rather than me.
"But what is absolutely clear is with me what you get is a renegotiation to address those issues that most matter to Britain, to make sure we have a European Union where you can be in the single market but not in the single currency, to deliver those objectives and then, come what may, whatever you think about this renegotiation, it will be the choice of the British people whether to stay in that reformed European Union or leave.
"That will be the real choice at the next election and, indeed, voting for Ukip is really only likely to help deliver a Labour government that won't give you a renegotiation and won't give you a referendum."
Mr Carswell's exit from the Commons was confirmed as Chancellor George Osborne appointed him to the role of steward and bailiff of the manor of Northstead, the formal way for him to leave.
The former Clacton MP will stand for Nigel Farage's party in the forthcoming by-election, and Mr Wheeler indicated that other Tory backbenchers could follow Mr Carswell to Ukip.
Mr Wheeler told Sky News he had taken a group of Tory MPs to a restaurant to woo them.
He said: "I didn't say 'Would you defect?'. I would say 'Would you like to meet Nigel Farage?'. Some of them said yes and obviously Douglas Carswell was one of them."
On the prospect of Tory defections increasing Labour's chances next year, he claimed Ed Miliband's party was "terrified" of Ukip.
He said: " There are a very large number of seats, particularly in the north, where we are second behind Labour where the Conservatives have no chance of winning it, where it would be greatly to the advantage of the Conservatives ... if Ukip wins it rather than Labour."
Mr Farage and Mr Carswell went on a joint walkabout in Clacton following the former MP's bombshell announcement.
The Ukip leader said: "I can't think of anybody else being honourable enough not only to defect on principle but also to put their own position at risk in a by-election."
Mr Carswell d efended his decision to quit, saying: " I stab people in the front, not the back."
He added: "I like David Cameron, he's a nice guy, he's actually good fun.
"I don't think I've ever stabbed anybody in the back - maybe I occasionally stab one or two people in the front, but I've been frank and straight with people.
"But I think he's not serious about change in Europe and I've put my political career on the line."
The visit attracted a mixed response from passers-by, with one woman shouting at Mr Carswell: "The traitor's blocking the street - stop blocking the street, traitor."
But several cars honked their horns in a show of support and taxi drivers pulled over to praise Mr Carswell.
Earlier, Roger Lord - the man who had previously been chosen to represent Ukip in Clacton at the general election - complained that he had been poorly treated.
He accused Mr Carswell of viewing his new party as an extension of the Conservatives and of defecting because the water was "around his ankles".
Mr Farage said he had already contacted Mr Lord and would speak to him again.
He said: "I would have thought that, if Douglas wins the by-election, Ukip will stand a very good chance in other seats in Essex.
"I know Roger well - he is loyal to the party and has a lot to offer. He can certainly be part of our future success in the county."
The date for the by-election is yet to be set but is likely to be held within weeks.
Parliamentary rules require an election between 21 and 27 days after a writ is moved to fill the vacancy.