David Cameron has had a "full and frank discussion" with the president of the European Council ahead of an expected clash at a Brussels summit over the nomination of Jean-Claude Juncker to the EU's top job.
The Prime Minister spelt out to Herman van Rompuy that he is determined to fight "until the end" the appointment as president of the European Commission of the former Luxembourg PM, who Britain regards as an ultra-federalist and a roadblock to reform.
Mr Cameron made clear that if Mr Juncker's name is tabled on Friday, he will demand a vote on his nomination - an unprecedented move in a process which is normally agreed by consensus.
Following their meeting, Mr Cameron wrote on his Twitter feed: " I had a full and frank discussion with Herman van Rompuy. I told him there must be a vote on Commission president."
Following the 40-minute meeting No 10 said Mr van Rompuy had agreed to work through the practicalities of holding a vote after Mr Cameron made clear that he was not prepared to back down, despite facing almost certain defeat.
With a majority of EU leaders - including Germany's Angela Merkel - throwing their weight behind Mr Juncker's candidacy, the Prime Minister looks certain to be outvoted under the qualified majority system, and is thought unlikely to secure enough support even to form a blocking minority of four or more member states.
Downing Street made clear Mr Cameron regarded it as a matter of principle, with the so called "Spitzenkandidaten" process - which would see Mr Juncker chosen as the candidate of the biggest party in the European Parliament - marking a fundamental shift in the balance of power in Brussels.
"The Prime Minister explained that his view would not change. Simply accepting the 'Spitzenkandidaten' process would be an irreversible step which would hand power from the European Council to the European Parliament, with the risk that the European Parliament would dictate the European Union's agenda," a No 10 spokesman said.
"It would also politicise the European Commission and compromise its exercise of its important regulatory functions. It would end the decades-long practice of always finding a candidate by consensus. And it would ignore the clear pro-change and pro-reform message delivered by European voters in the recent European Parliament elections."
Mr Cameron's intervention underscored the divisiveness of the decision, amid widespread speculation that leaders including Mrs Merkel are privately unhappy with the nomination. Chancellor George Osborne accused European leaders of saying one thing in private and another in public about the appointment.
"I think there is a fairly odd phenomenon at the moment - which does happen, believe it or not, in politics - which is that people are saying quite a lot of things privately that they are not saying publicly," Mr Osborne told BBC Radio 4's Today programme.
"I think privately a lot of European governments understand that the institutions of Europe need reform, that they are under huge pressure from their domestic populations to deliver stronger economies, more investment to Europe, that Europe needs to better connect as a union to the people on this continent."
As chairman of Friday's meeting, Mr van Rompuy holds a key position in deciding whether the Juncker candidacy is put to a vote. The Council's rules of procedure do not specifically state whether a vote must be held if requested by a single national leader, but it is thought unlikely that Mr van Rompuy would reject Mr Cameron's demand.
After taking soundings on the Commission appointment in national capitals over the past month, he has to decide whether to press ahead with tabling a candidate who does not enjoy universal support among the member states or to ask them to take a little longer to try to find consensus.
There has been speculation that he would prefer to delay a decision to avoid a row over the nomination overshadowing a visit by EU leaders on Thursday to Ypres to mark the 100th anniversary of the start of the First World War.
Europe minister David Lidington suggested that the decision could be put off to a later meeting, telling the House of Lords EU Committee: "Even now one picks up reports of this or that leader suggesting that there might need to be further meetings rather than taking this decision at the Ypres summit."
It is thought that there is some concern within the Commission at the prospect of having a president who is opposed in public by at least one key member state and might not enjoy the private support of other capitals.
There are also believed to be worries at the Commission's Berlaymont HQ that a president who owes his position to the MEPs who chose him as their candidate might be too beholden to the European Parliament.
Deputy Prime Minister Nick Clegg, who met Mr van Rompuy separately, said he had underlined the cross-party opposition to the appointment of Mr Juncker, with Labour also against handing him the job.
"Mr van Rompuy can be under no doubt that the strength of feeling about making sure that the role of national governments - directly elected by each of the peoples of the European Union - their role should be protected and we shouldn't simply subcontract the decision about who becomes the European Commission president to MEPs, hook, line and sinker," he said.