DAVID CAMERON warned other European leaders they were making a mistake over Brussels' top job as he looked increasingly isolated ahead of a showdown vote today.

The Prime Minister is expected to face a humiliating defeat over the choice of the new European Commission President.

On the eve of the trip, Mr Cameron pledged he was at the start of a "long campaign" for European Union reform.

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He vowed not to back down on the issue of the next president, which could determine the outcome of his pledged 2017 In/Out EU referendum.

Labour have accused the Conservative leader of uniting other countries against the UK through his fight against Jean-Claude Juncker's appointment.

No 10 is furious at what sources say is the willingness of other European countries to back Mr Juncker in public, while expressing reservations about his suitability in private.

A few weeks ago Mr Cameron had hoped to rely on support from a number of other leaders, including German Chancellor Angela Merkel, against Mr Juncker.

Mrs Merkel has said it would be "no tragedy" if the UK was left isolated.

Downing Street insist Mr Juncker is the wrong candidate at the wrong time, an advocate of greater European integration when voters want change,

The stakes are incredibly high for Mr Cameron who has pledged to secure a new relationship with the EU before the 2017 vote.

Political observers predict that project will be made all the more difficult with Mr Juncker at the helm.

Earlier this week Downing Street refused to rule out that his appointment could lead UK ministers to back the UK leaving the EU.

No 10 sources have also moved to play down suggestions that the UK could get a better deal on renegotiation, following humiliation over Mr Juncker.

They said the matter of who becomes president is one of principle, not a bargaining chip.

Mr Cameron made a similar point before he met other European leaders yesterday and ahead of a working lunch today when Mr Juncker's potential appointment will be on the agenda.

The Prime Minister said he would not be swept off course as he attacked others for their duplicity and the decision to back Mr Juncker. He also appeared to appeal to UK voters that they could trust his word on Europe.

That move will be widely interpreted as a bid to head off Ukip, which appeared to gain traction in the recent European Parliament elections with their attack on Mr Cameron's record on the issue.

"People need to know that with me, when I say I'm going to do something in Europe, I'll do it," Mr Cameron said.

"When I said I was going to cut the European budget I did, I said I would veto a treaty if I wasn't happy with it I did, and I said I would oppose this and insist on a vote and I will."

Labour's shadow foreign secretary Douglas Alexander accused Mr Cameron of making a mistake.