David Cameron has warned fellow EU leaders of "consequences" if they press ahead with plans to nominate an arch-federalist, whom Britain regards as an obstacle to reform, as president of the European Commission.

The Prime Minister insisted he was "completely unapologetic" about his outspoken opposition to the appointment of former Luxembourg premier Jean-Claude Juncker, which has left him isolated at a two-day summit of the European Council.

Mr Juncker's nomination to the EU's top job is expected to be confirmed in an unprecedented vote in Brussels today, breaking a decades-old tradition that the Commission chief is chosen by consensus of the EU's national heads of government.

But Britain insists that its dissatisfaction with Mr Juncker is "not a unique view" and that privately other capitals have misgivings about his candidacy.

Amid widespread reports that Mr Juncker's liking for "a cognac at breakfast" were causing concerns in Brussels, one European diplomat said: "His alcohol consumption has been raised by a number of leaders since the (European) parliamentary elections."

With Chancellor Angela Merkel throwing Germany's weight behind Mr Juncker - despite reportedly assuring Mr Cameron initially in private that he would not get the job - and other potential allies such as Sweden and Netherlands also dropping their opposition, only Hungary remained as a possible partner for Britain in voting against his installation.

But Mr Cameron said he continued to believe his fellow leaders were making a "mistake" and that choosing Mr Juncker - candidate of the largest political grouping in the European Parliament, the centre-right EPP - would be "bad for all of Europe".

In an apparent swipe at leaders who have voiced disquiet behind closed doors, Mr Cameron said: "It's very important in Europe that you say what you say in private and it's the same as what you say in public."

Asked whether there could be consequences if the other 27 leaders refuse to accept the need for consensus, the Prime Minister said: "Everything has consequences in life.

"Obviously, I think proceeding in the way that countries are planning to proceed in choosing this individual, I believe that this is the wrong approach. And I think that would be bad for... all of Europe."

Mr Cameron made clear that defeat in Brussels would not affect his determination to press ahead with renegotiation of Britain's EU membership, followed by an in/out referendum in 2017, if Conservatives win next year's general election.

"Does any of this mean that we do not get a renegotiation? No," he said. "Does it mean we don't get a referendum? No."

But it is understood that the Prime Minister believes that a refusal on the part of the European Council to take account of the reservations of a large member of the EU - and a net contributor to its budget - will have an impact on the mood in which the referendum debate is conducted in the UK.

One British official said: "We are realistic that that has an impact on the debate in Britain about the EU. That's what other leaders must realise... What we have been saying to people is: 'You need to realise that there is a debate going on in Britain about Britain's role in the EU. It's not one prime minister, leading one party, with a view. There is a broad debate here, and you need to understand that political reality.'"

Victory for Mr Juncker would "make securing reform and securing renegotiation even more important and will make us even more determined", said the official.

Mrs Merkel appeared to offer an olive branch to Mr Cameron as she arrived for talks with fellow centre-right leaders ahead of the summit, saying that there was room for a "good compromise for the UK" in a document setting out the EU's strategic agenda for the next five years.

This consolation prize could involve strong words in the document - due to be signed in Brussels today - on British priorities such as jobs, growth, deregulation, immigration and "welfare tourism", which might act as a constraint on the actions of an eventual Juncker Commission.

British officials were playing down speculation that agreement on the Commission presidency might be reached through horse-trading over other top jobs - including the offices of European Council president and High Representative for foreign affairs, which also come up for renewal in November - insisting that Mr Cameron will make good on his promise to maintain his opposition "until the end".

Britain is angling for one of the key economic portfolios at Berlaymont, but decisions on individual commissioners are not expected until a later summit next month.

The prime ministers and presidents of the 28 EU states yesterday set aside their differences on the first day of the summit to commemorate the 100th anniversary of the outbreak of the First World War in a ceremony at Ypres, scene of some of the conflict's bloodiest battles.

After a minute's silence at the Menin Gate - where the name of one of his great-great-uncles is inscribed among the war dead - Mr Cameron told the other leaders at dinner that the event reminded him of the central role Britain played in Europe's past and said that he was committed to ensuring it played a central role in its future.

In a reminder of his call for the goal of "ever closer union" to be scrapped, he stressed the need to respect the role of nation states within the EU and to recognise that not all of them are heading towards the same destination.

Labour accused Mr Cameron of mishandling the issue by "playing the man, not the ball".

Shadow foreign secretary Douglas Alexander told the BBC yesterday: "Sadly, the handling of these negotiations have turned a Europe divided over Jean-Claude Juncker into a Europe apparently united against David Cameron.

"What we needed was to work to build an alliance with countries that were themselves unconvinced by Jean-Claude Juncker's candidacy - countries like Sweden, Denmark, Holland, Hungary, Italy. There was an alliance there to be built, but alas it appears that the Prime Minister so badly misjudged his tactics and strategy that that is not going to be the outcome in the next 24 hours."

Arriving at the Council building in Brussels for the start of the summit's second day, Swedish Prime Minister Fredrik Reinfeldt confirmed he was backing Mr Juncker's candidacy, though he said he shared Mr Cameron's criticisms of the "Spitzenkandidaten" process, in which political groupings in the European Parliament put forward their choices for the Commission presidency ahead of last month's elections.

Mr Reinfeldt echoed Mr Cameron's concern that Mr Juncker's selection might fuel Euroscepticism in the UK ahead of a referendum, but said the British PM should be reassured by the reformist tone of the strategic agenda being signed in Brussels, which he said was more important than the identity of individuals taking the top jobs.

Asked if he understood Mr Cameron's worries about the Juncker nomination, Mr Reinfeldt said: "I share the concerns about how this should be interpreted in Britain, because it is a core value for us that Britain stays inside the European Union - then we get a politically and economically stronger European co-operation. It's also in the best interests of the United Kingdom, that is my belief.

"I hope that he is happy with the programme itself. We discussed it at a meeting in Sweden, looking at conditions for trade, for free trade agreements and competitiveness, where we are very like-minded. The programme in itself is more important than the persons elected.

"He and the British people have friends inside the European Union who like British politics, like British people and want them to stay inside the European Union. Maybe we should voice that a little bit more clearly so that they hear us."

Belgian PM Elio di Rupo said the EU leaders should "respect the citizens" who voted in the European elections to make the EPP the largest grouping in the European Parliament.

Asked about British opposition to Mr Juncker, he said: "Their point of view is respectable, but they can't on their own block 26 or 27 other countries who are agreed.

"There is a very, very large majority at the heart of the EU and the majority of countries want to go ahead."

Mrs Merkel made no comment as she arrived at the summit.

Asked whether defeat for Mr Cameron could make a British exit from the EU more likely, Finnish Prime Minister Alexander Stubb said: "I think in the UK some people really seriously need to wake up and smell the coffee.

"The EU is a very good thing for the UK. Over 50% of the trade of the UK goes to the EU. If that were to be cut off, I think the continent would be cut off, seriously."

Mr Stubb added: "I think David Cameron is a very principled man and he has felt strongly that Juncker is a little bit too federalist for his liking, but I think we can all make amends, we can all solve the situation."