David Cameron has reportedly warned that Britain could leave the EU if Luxembourg ex-prime minister Jean-Claude Juncker becomes the next president of the European Commission.
German news magazine Der Spiegel claims the Prime Minister issued the warning to German Chancellor Angela Merkel at the EU leaders summit in Brussels on Tuesday.
Quoting sources close to those at the summit, the magazine said Cameron fears the appointment of continuity candidate Juncker would destabilise the UK Government to such an extent that it would bring forward an in-out EU referendum.
Cameron reportedly added: "A figure from the Eighties cannot resolve the problems of the next five years."
A Downing Street spokesman said: "We are not commenting on this. It was a private meeting, a private conversation."
Cameron has previously made it clear he views the former Luxembourg leader as a symbol of Europe's past and has argued that a reformer should take charge of the EU executive to pave the way for change.
However, on Friday Ms Merkel gave her backing to Juncker for the key post, putting her and the Prime Minister on a collision course.
Both Merkel and Juncker's parties are members of the European People's party (EPP) bloc, which still dominates the parliament and has selected him as its preferred candidate.
It comes as Iain Duncan Smith said Cameron knows he must deliver "significant return of powers" from the EU to win the support of Tory colleagues in a referendum on British membership.
The Work and Pensions Secretary said the PM was clear he could not "come back with nothing" from a mooted renegotiation after the general election.
He also lashed out at the BBC for failing to give enough prominence to Cameron's promise of an in-out vote in 2017.
Although Duncan Smith stressed he was "happy" with the premier's stance, the comments will heap pressure on Cameron to flesh out his ambitions for repatriating powers from Brussels.
Speaking to the Sunday Telegraph, Duncan Smith said: "I think you will find that the Prime Minister in due course will be a little clearer about some of the stuff that he thinks.
"The key thing is he knows very well that he can't come back with nothing.
"I don't think the Prime Minister in any way is lost on that fact that he will need to get back some substantial and significant return of powers. It is for him to deliver that."
Duncan Smith called for new rules to limit migration from the EU, and said Brussels should be stripped of control over who is entitled to state benefits in Britain.
He said he challenged Cameron face-to-face to confirm his commitment to delivering a referendum after the next election.
"I have looked him in the eye and I have asked him the simple question: you are absolutely clear this referendum is going to happen if the Conservatives are back in power and he said 'Yeah, I won't be in a government if they won't have a referendum.'"
Speaking on the by-election campaign trail in Newark, he signalled that if Cameron failed to get a good deal, he would be ready to vote to leave the EU as Britain could "prosper" outside the union.
"Let's have that referendum and when the Prime Minister goes and does his negotiations, if I am lucky enough to be in government at the time, I will do my level best to help him and support him.
"When he comes back I will make a decision on that like everybody else in Britain will do."
Duncan Smith attacked the national broadcaster for failing to cover Tory policies properly.
He warned that "most people" do not even know about the Conservatives' pledge to hold an in-out referendum on EU membership.
"The BBC has spent their whole time downgrading this whole pledge," the Cabinet minister said.
The broadcaster's executives take the view that "we've done that" and the policy does not need explaining, he said. "They don't do the British people a service at all.
"The British people use the BBC more than anything else to find out what's going on in politics. When was the last time you really heard this?"
Duncan Smith said public ignorance of the referendum plan was "the challenge to us over the next months to the election".
However, he argued Ukip's strong showing in local and European elections on May 22 was simply a "protest" which would fade away by polling day next year.
The former Tory leader also insisted the party had shown "some significant discipline when compared with the other two parties" in the wake of the results.
"I was leader once upon a time, when frankly you couldn't hold a raffle in this bloody party but you would have 50% of them against and 50% of them for," he said. "So for the Conservatives to show discipline is significant."
A BBC spokeswoman said: "The BBC has given the Conservative Party's policy on an in-out referendum on Europe extensive coverage from the moment the pledge was made, and ever since.
"We are satisfied that we have covered the EU referendum promise impartially and accurately."
Shadow foreign secretary Douglas Alexander sought to turn up the heat on Cameron by demanding he spell out his negotiation aims.
In a letter to the PM, Alexander wrote: "It is now 16 months since your Bloomberg speech about the future of Europe where you described five key principles that you said should underpin the way Europe should change: competitiveness, flexibility, power flowing back to nation states, accountability and fairness.
"At the time, there was widespread agreement in the UK that, while abstract, these five principles were indeed important ideals.
"Since then, despite repeated requests from leading members of the British business community, from within your own party and from MPs from all sides, you have failed to set out your specific reform agenda for the EU.
"The concern over your silence is widely held. Only this week, dozens of Conservative activists, MPs and senior figures from the grassroots of your party, called on you to urgently offer 'clarity' on your European reform agenda which, they point out, you have conspicuously not done to date."