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.
More top news
Ursula Keogh, 11, went missing on Monday around 3.30pm. She was dressed in her school uniform.
"My sincerest apologies to everyone who purchased tickets and were planning to come to the upcoming shows."
His comments at the Defence and Security Forum on Monday came after a £20 billion black hole in the budget for the next decade emerged.