David Cameron is said to be ready to pull Britain out of the EU if federalist Jean-Claude Juncker becomes president of the European Commission.
The Prime Minister is alleged to have issued an ultimatum to German Chancellor Angela Merkel at a summit in Brussels.
German magazine Der Spiegel reported that Mr Cameron considers the ex-Prime Minister of Luxembourg too destabilising for the British government for him to work with.
Mr Juncker is the among the frontrunners to take on the key post - and enjoys the support of Ms Merkel.
But Mr Cameron is said to have told her: "A figure from the Eighties cannot resolve the problems of the next five years."
The Prime Minister is known to want a reformist candidate to take over the presidency in order to enable reform.
Ms Merkel and Mr Juncker both belong to parties in the European Peoples' Party (EPP) bloc in Brussels.
After Mr Cameron's comments were reported, Mr Juncker took a thinly-veiled swipe at the Prime Minister, telling Germany's Bild leaders should not be "blackmailed" into rejecting his presidential bid.
A Downing Street spokesman said: "We are not commenting on this. It was a private meeting, a private conversation."
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.
