David Cameron today received a welcome boost in his battle with Conservative rebels over Europe, as US president Barack Obama backed his strategy of seeking a renegotiation of Britain's membership of the EU before holding a referendum on withdrawal.
Speaking after talks with the Prime Minister in Washington, Mr Obama said it "makes some sense to me" to try to "fix" the relationship before deciding whether to leave.
His comments, which delighted Downing Street aides, came as growing numbers of Conservative MPs signed up to a parliamentary motion calling for legislation on an EU referendum, which is expected to spark a potentially divisive vote in the House of Commons on Wednesday.
Two serving Cabinet ministers, Education Secretary Michael Gove and Defence Secretary Philip Hammond, yesterday said that they would vote to leave the EU if there were a referendum today. Ministerial aides Stuart Andrew and Gavin Barwell said they will back the amendment after Downing Street took the unusual step of saying that parliamentary private secretaries (PPSs) are free to do so without losing their jobs. Ministers have been told they may abstain but not vote for the amendment.
Mr Cameron said he regarded the status quo in Europe as "unacceptable", but rebuked those - like Tory grandees Lord Lawson and Michael Portillo - suggesting the UK should pull out now.
"The point I would make to those people is you should not give up before a negotiation has started. It seems to be an extraordinary way to go about things," the Prime Minister said in a broadcast interview.
"The idea of throwing in the towel before the negotiations have even started, I think, is a very, very strange opinion."
He insisted that "the whole of the Conservative Party" was signed up to his policy of renegotiating if he wins the 2015 general election and then staging a national poll in 2017, adding: "We are the only mainstream party making this offer to the electorate at the next general election."
At a White House press conference, Mr Cameron insisted that a referendum now would not be in the national interest, as voters would be presented with a "false choice" between an unreformed EU or British exit.
"Is it in our interests to reform the European Union, to make it more open, more competitive, more flexible, and to improve Britain's place within the European Union?" he asked. "Yes, it is in our national interests. And it's not only in our national interest, it is achievable.
"Is it in Britain's national interest once we have achieved those changes, but before the end of 2017, to consult the British public in a proper, full-on in/out referendum? Yes, I believe it is.
"That's the approach we take. Everything driven by what is in the British national interest. That is what I'm going to deliver. It's absolutely right for our country. It has very strong support throughout the country and in the Conservative Party and that's exactly what I'm going to do."
Mr Obama stressed that the decision on the UK's future was a matter for the British people, but he added: "David's basic point that you probably want to see if you can fix what is broken in a very important relationship before you break it off makes some sense to me.
"I know that David's been very active in seeking some reforms internal to the EU. Those are tough negotiations, you've got a lot of countries involved, I recognise that.
"We haven't yet evaluated how successful those reforms will be. I at least would be interested in seeing whether or not those are successful before rendering a final judgment."
Mr Obama's comment earlier this year that the US valued "a strong UK in a strong European Union" was widely interpreted as an indication of concern in Washington that Britain was heading towards withdrawal.
Today, the president said: "I think the UK's participation in the EU is an expression of its influence and its role in the world as well as obviously a very important economic partnership."
At least 53 MPs have signed the rebel amendment, and eurosceptic backbencher Peter Bone today predicted that "most" Conservative MPs will vote for it on Wednesday.
Mr Bone told BBC2's Daily Politics: "The amendment ... is of course the Prime Minister's policy. I am sure that in America, he is toasting what I and a number of other colleagues have done, and I'm sure that if he wasn't in America he would be supporting the amendment.
"Most Conservative MPs will vote for it. By the time we get to Wednesday, I hope the ministers will be allowed to vote for it."
Tory MP John Baron, who tabled the amendment with Mr Bone, urged the Prime Minister to support it if it is selected for debate by Speaker John Bercow.
"It provides the PM with a golden opportunity," said Mr Baron. "He can legitimately claim this situation was not of his making, and therefore not confined by the coalition agreement. He has the freedom to do the right thing for the country should he choose to do so. He should encourage his party to support the amendment, and then introduce legislation in Government time."
Senior Conservative Nicholas Soames accused those who argue that legislation is needed to ensure Mr Cameron makes good on his promise of a post-2015 referendum of "chipping away at the Government's integrity".
Urging MPs and ministers to be "a little cautious in trivialising what is involved", Mr Soames said: "This a hugely important decision for this country, probably the most important that it will have to take for generations, and it is not to be lightly taken or on the basis of prejudice or pub rhetoric.
"No good is done to the public governance of this country in this constant chipping away at trust and at the Government's integrity. If the Prime Minister says something will happen, such is the momentous nature and importance of this decision, it will happen with orderly process and proper debate and not in some hysterical, knee-jerk publicity-seeking act."
Labour Europe spokeswoman Emma Reynolds said: "It beggars belief that the Prime Minister, who is leader of the Government, is almost encouraging - or apparently 'relaxed' about - his own MPs voting against his Government's programme."
A senior Labour source confirmed the party would oppose Mr Baron's amendment next week.
The source said: "We do not think committing now to an in/out referendum on Europe is in the national interest. We will vote against the John Baron amendment."