Prime Minister David Cameron insisted that 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".
Mr Cameron 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," said the Prime Minister.
"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."