David Cameron has insisted he welcomes the renewed focus on Conservative European policy sparked by former chancellor Lord Lawson's public call for the UK to quit the EU.
Lord Lawson's intervention, in an article in The Times, has electrified Tory backbenchers, with one suggesting the PM could offer a coalition deal with the eurosceptic UK Independence Party and the post of deputy prime minister for its leader Nigel Farage.
Mr Farage claimed the Tory peer's comments legitimised his party's anti-EU position and exposed Conservative divisions on the issue, while Liberal Democrat leader and deputy prime minister Nick Clegg said the Tories were "struggling" to work out how to deal with Ukip.
Lord Lawson, who was Margaret Thatcher's longest-serving chancellor and remains a highly-respected figure within the party, said there was now a "clear" case for UK withdrawal from the EU, insisting that the economic benefits would "substantially outweigh the costs".
The prime minister has promised to renegotiate the UK's relationship with the EU if Conservatives win the 2015 general election, leading to an in/out referendum vote by 2017.
But in the wake of Ukip's surge in last week's county council elections, there is also pressure to put the strategy to a vote in the Commons in defiance of his Liberal Democrat coalition partners or to hold a "mandate referendum" before the election to secure public backing for renegotiation.
Speaking at a press conference in the Foreign Office, Mr Cameron played down suggestions that Lord Lawson's comments had been unhelpful in stoking up Tory debate on Europe, which has previously been split by the issue.
"I think it has been a good day for the pledge that I have made that, if re-elected, I will hold an in/out referendum so that everybody can have not just a voice about Britain's future in Europe but also have a vote about Britain's future in Europe," he said.
"I welcome the attention that has been placed on this key pledge that I have made. Only a Conservative government, only the Conservatives of the major parties, are making this promise.
"I want to give people a choice not between the status quo and leaving the EU. I want to give people a choice between Britain remaining in a reformed EU or leaving that EU. That is the choice that people want and there is only one way to get it and that is by supporting the Conservatives at the next election."
Mr Cameron appears to have ruled out legislation in this Parliament to pave the way for the referendum, in response to a petition signed by 100 Conservative MPs.
Tory backbencher John Baron, who organised the petition, said that a reply he has received from Mr Cameron stated that the prime minister was prevented from doing so by the coalition agreement with Liberal Democrats.
The Basildon and Billericay MP said the prime minister's reply said that the government's legislative programme depends on the coalition agreement which "did not include legislation in this Parliament for an in/out referendum".
But Mr Baron said he would continue to press his case and would be willing to introduce a private member's bill to enshrine Mr Cameron's referendum pledge in law.
"My hope is that if the government cannot introduce legislation, it will at least support legislation if it is introduced via other means," he said.
"My message to the prime minister is that he needs to support our calls for legislation in this Parliament for a referendum in the next. This would show Conservatives are serious in their commitment. There is no downside whether successful or not. It would be good for the country, and reduce Ukip's appeal."
Lord Lawson was dismissive of the prospects for Mr Cameron achieving significant reform in his planned renegotiation, claiming it would only secure "inconsequential" concessions.
But he said that the creation of the single currency meant that the case for British exit was now "clear".
"The heart of the matter is that the very nature of the European Union, and of this country's relationship with it, has fundamentally changed after the coming into being of the European monetary union and the creation of the eurozone, of which - quite rightly - we are not a part," wrote Lord Lawson.
"That is why, while I voted 'in' in 1975, I shall be voting 'out' in 2017.
"Not only do our interests increasingly differ from those of the eurozone members but, while never 'at the heart of Europe' (as our political leaders have from time to time foolishly claimed), we are now becoming increasingly marginalised as we are doomed to being consistently outvoted by the eurozone bloc.
"So the case for exit is clear."
While there would be "some economic cost" from leaving the EU single market, he went on, "in my judgment the economic gains would substantially outweigh the costs".
Downing Street said the prime minister remains "confident" that his strategy "will deliver results".
Lord Lawson also hit back at the deputy prime minister who told BBC Radio 4's Today programme that leaving the EU would make the country "less prosperous" and would jeopardise "up to 3 million jobs".
The former chancellor retorted: "That's poppycock, I don't think Nick Clegg, who is a charming young man, has ever purported to know anything about economics."Suggest a correction