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
Wonderfully sunny weekend
Singer Louis Tomlinson wished his mum a happy birthday, just months after her death from an aggressive form of leukaemia.
The multi-million pound company wanted 500 teenagers to work unpaid - but the chain changed its mind after being criticised on social media.