The Prime Minister has "indirectly threatened" Germany's Angela Merkel that he would no longer guarantee British membership in the EU if European leaders elect Jean-Claude Juncker as European Commission chief, German magazine Spiegel said.
David Cameron said that if Mr Juncker became the EU Commission's president, the UK government could be destabilised to the extent that an "in-out" referendum would have to be brought forward, Spiegel reported.
The European Commission president is selected by EU leaders but must be approved by the EU parliament where Eurosceptics from the right made gains in last week's election.
Nigel Farage has claimed Ed Miliband will be forced to back calls for an automatic EU referendum if Ukip outperform Labour in the upcoming European elections.
The Ukip leader told the BBC's Andrew Marr Show there was growing Labour backbench support for holding an in-out referendum, despite the Labour leadership clearly opposing it.
Mr Miliband told the same show that Labour would back a referendum if powers were transferred from Britain to the European Union, a situation he judged "unlikely but ... possible".
But he said debating whether we should exit the EU was not "a priority" among his plans to govern Britain.
Mr Farage, meanwhile, refused to be drawn on whether he would run as an MP at the next General Election after opting against a candidacy in the upcoming Newark by-election.
"I want Ukip to win the European Elections," he said. "We'll talk about the General Election afterwards."
Almost half of Britons over the age of 50 would vote to leave the European Union, a new survey reveals.
In a nationwide poll of 11,211 people aged 50 and over, 45 percent said they would vote to leave the EU.
Nearly two-thirds (66 percent) would prefer to have a referendum on EU membership before the next general election, the survey conducted on behalf of Saga showed.
Conservative MPs proved they can agree on the European Union, or at least on one aspect of policy.Read the full story ›
This in itself doesn't guarantee it [a vote in the next Parliament], although of course if we got his onto the statute book and made it law then it's much harder for other parties to turn against it in the future.
And the Bill we have passed two years ago - the Act of 2011 now requires a referendum if any British government gives away more powers to the European Union now has been accepted by all the other parties.
Tory MP James Wharton's European Union (Referendum) Bill, which would force a public vote by 2017 on whether the United Kingdom should remain within the EU, was unanimously given a second reading in the House of Commons
The closure motion was approved by 305 votes to 30, majority 275. Another vote was called on whether to give the EU Referendum Bill a second reading at 2.15pm.
Chief Whip Sir George Young brought a closure motion at 2.04pm. It was opposed by a number of Labour MPs and a vote was called.