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 Mr Cameron fears the appointment of continuity candidate Mr Juncker would destabilise the UK Government to such an extent that it would bring forward an in-out EU referendum.

Mr 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."

Mr 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 Mr Juncker for the key post, putting her and the Prime Minister on a collision course.

Both Mrs Merkel and Mr 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 Mr 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 Mr Cameron's promise of an in-out vote in 2017.