British Prime Minister David Cameron has warned he would no longer be able to guarantee that Britain would remain a member of the European Union if European leaders elect Jean-Claude Juncker as European Commission chief, Germany's Spiegel said.
The European Commission president is selected by EU leaders but must be approved by the assembly, where Eurosceptics from the right made gains in last week's election. The European People's Party, which won the most seats in the vote, had chosen Luxembourg's ex-premier Juncker as their candidate.
In a pre-publication copy of an article, Spiegel said Cameron had explained, on the sidelines of an EU summit in Brussels on Tuesday, that if Juncker became Commission president, he would no longer be able to ensure Britain's continued EU membership.
The magazine said participants understood Cameron's comments on the sidelines of the meeting to mean that a majority vote for Juncker could destabilise his government to the extent that an "in-out" referendum would have to be brought forward.
That in turn, they understood, would most likely lead to the British people voting to quit the EU, it said.
The magazine said Cameron, who regards Juncker as too federalist and likely to damage his hopes of reforming Britain's EU ties, dismissed the candidate during a recess with the words: "A face from the 80s can't solve the problems of the next five years."
A spokesman at the prime minister's office declined to comment on the Spiegel article.
Cameron has promised to renegotiate the terms of Britain's EU membership and if his Conservatives win a 2015 national election to hold a referendum by the end of 2017 on whether or not to remain in the EU.
On Monday he rejected calls to bring the referendum forward after his party was beaten into third place in European elections by the anti-EU UK Independence Party (UKIP) party.