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 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 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 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 irrespective of any renegotiation that I manage to complete. I was pretty confident Douglas Carswell was one of those people.
"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 EU where you can be in the single market but not in the single currency, to deliver those objectives and then... it will be the choice of the British people whether to stay in that reformed EU or leave.
"That'll be the real choice at the next election... 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."