David Cameron is on a collision course with Angela Merkel after the German chancellor backed a continuity c andidate as the next president of the European commission.
Mrs Merkel announced she wants Jean-Claude Juncker to get the key post despite the Prime Minister making clear he views the former Luxembourg leader as a symbol of Europe's past.
Mr Cameron was thought to have won over some of his counterparts at a Brussels summit this week, where he argued that a reformer should take charge of the EU executive.
He insisted that recent elections, which saw strong performances by Eurosceptic parties across the continent, demonstrated the need for change.
But speaking at the National Catholic Congress in Regensburg, Mrs Merkel is reported to have said: "I will now lead all negotiations in the spirit that Jean-Claude Juncker should become president of the European commission."
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.
However, previously she had stopped short of endorsing him for the job, saying "anything is possible".
Mr Cameron and his Hungarian and Swedish counterparts were among those rejecting demands for Mr Juncker's appointment to be rubber stamped in Brussels this week.
They insisted it was up to national leaders on the European Council to propose a new president, rather than the parliament.
"Europe cannot shrug off theses results. We need an approach that recognises that Europe should concentrate on what matters, on growth and jobs and not try and do so much," Mr Cameron said at the summit.
"We need an approach that recognises that Brussels has got too big, too bossy, too interfering. We need more for nation states. It should be nation states wherever possible and Europe only where necessary.
"Of course we need people running these organisations that really understand that and can build a Europe that is about openness, competitiveness and flexibility, not about the past."
Meanwhile, it has been confirmed that Mr Cameron will hold bilateral talks with US President Barack Obama at the gathering of G7 leaders in Brussels next week.
Downing Street said their discussions were expected to cover topics including the special relationship, global economic situation and foreign policy issues. The two leaders last met at the G7 meeting in The Hague in March.