David Cameron has said he will quit as prime minister if he cannot deliver an in-or-out referendum on the EU after the next general election.
Cameron made the commitment in a conference call to party supporters on Monday evening, saying if he remained prime minister after the next election but could not deliver the referendum by 2017, he would leave office.
Promising the EU referendum is the main thrust of Cameron's effort to stop Tory voters defecting to Ukip, which is polling in first place, leaving the Conservatives in third, ahead of May's European elections.
Ukip leader Nigel Farage has said Cameron will not keep the pledge and has only said it to improve his party's chance of winning the next general election.
Cameron said he would not “barter away” the referendum in any new coalition negotiations that might follow the election if there is still a hung parliament.
“This is not something I would ever barter away or give away. I would not be prime minister of a government unless we could carry out our pledge of an in-out referendum,” Cameron said.
“I’ve already pre-empted that question, what happens if somehow you don’t win the election outright. I’ve been very clear: I would not continue as prime minister unless I be absolutely guaranteed this referendum will go ahead on an in-out basis. I can’t make it any clearer than that.”
He added: “People are always sceptical about our policies but the more times we say it, the more times we put it on our leaflets, on our posters, the more the British people will see it’s absolutely a guaranteed part of our plan."
During the conference call, one Conservative activist and small businessman from London, told him: "I don't doubt your sincerity about the commitment to have a referendum, I just don't think you are going to be allowed to negotiate sufficient changes to satisfy the British public and people like me."
He added: "They are simply not going to give you sufficient room to change the rules just for the UK."
Matthew Elliott, chief executive of Business for Britain, said: "The prime minister is right to make a referendum a red line for any future government of his, especially if he wants British business to prosper in years to come.
"Britain will only be set free from the status quo and get a better deal from the EU if negotiations for reform are backed up by the guarantee of a referendum.
"Those in Westminster who talk the talk of reform must now walk the walk and commit to giving Britain a vote in 2017 - just as polling shows British business wants."
Shadow cabinet minister Michael Dugher - who along with Labour strategy director Greg Beales makes up the party's negotiating team - said Cameron was scared of entering the fray.
"I can see why Cameron doesn't fancy it; he is like a fighter with a glass jaw with his record of failure, broken promises, standing up for the wrong people.
"But that's not good enough," he told Radio 4's Today programme.
"Ed Miliband has made his position absolutely clear about the importance of the TV debates, about his determination to get on with them. We are all waiting for Cameron.
"We know why he is not keen but frankly these should not be in his gift. They belong to the public and we should get on with them."