David Cameron will risk inflaming the row with fellow EU leaders over his wish to block Jean-Claude Juncker from taking the European Commission presidency by forcing an unprecedented vote on the issue this week.
The prime minister will spell out his concerns in a meeting with European Council president Herman Van Rompuy on Monday, ahead of a meeting with counterparts from across the 28-member bloc on Friday.
He wants a delay in the process in an effort to find a consensus candidate, but if leaders are not even willing to consider alternatives to Juncker, the prime minister will call a vote and require his counterparts from across the EU to set out their positions clearly.
Cameron sees Juncker as a road block to the reforms he seeks in an effort to woo back Tory voters who have defected to eurosceptics Ukip, who triumphed in last month's European elections alongside other EU parties across the continent.
The move would mark a distinct break from the way that the commission president is usually chosen, with the nomination agreed between the leaders.
The prime minister's decision to oppose Juncker's nomination looks set for failure, after nine centre-left leaders, including French president Francois Hollande and Italy's Matteo Renzi declared their support for Juncker - the candidate nominated for the presidency by the centre-right European People's Party (EPP).
Juncker, viewed as an arch-federalist by Cameron, was put forward by EPP, the largest party in the European Parliament after last month's elections, under the "Spitzenkandidaten" process.
Number 10 sources said in the past there had always been an effort to find a consensual candidate, as in 2004 when the UK did not force through Lord Patten as commission president because France were vehemently opposed, even though he had sufficient support from other countries for the required qualified majority vote.
Explaining why Cameron was prepared to take the step of forcing a vote, a source said his EU counterparts should be required to explain why they were "handing power to the (European) parliament through a back-room deal".
Cameron has vowed to "fight this right to the very end" but his stance was criticised by Austrian chancellor Werner Faymann, one of the centre-left leaders who agreed to back Juncker at the Paris summit.
He is reported to have told the Kleine Zeitung newspaper that Cameron could be defeated at the European Council summit if he forces the issue to a vote and added: "We cannot allow a single person to dictate everything to us."
Poland's foreign minister Radoslaw Sikorski told Sky News' Murnaghan programme that the process for nominating Juncker was democratic, because he was the choice of the EPP.
"In Dublin a couple of months ago, we agreed that this time it would be more democratic, not just national leaders meeting in a room overnight and coming out with a solution - this time we decided to nominate a candidate ahead of the election.
"And the candidates of the socialists, the liberals, and the centre right were plastered all over Europe - so those who bothered to take an interest knew that if they voted for us, then Mr Juncker would be the candidate. And we won - we got the largest number of votes.
"And so we - as the European People's Party - are having the first shot at putting together a team that will create a coalition. The winning party's top candidate is the candidate for the top job - that's democracy."
But he added: "I think there is still every room for influencing Mr Juncker's programme, and Mr Juncker's team - including the position of Britain's commissioner."
The need for Cameron to be able to negotiate a new arrangement with Brussels was underlined by an opinion poll which showed that almost half of Britons would vote to leave the EU under the current membership terms.
The Observer/Opinium poll found that 48% of people would "definitely" or "probably" vote to leave the EU, while 37% said they would definitely or probably vote to stay in.
Cameron has vowed to renegotiate the UK's relationship with Brussels before an in/out vote on membership of the EU by the end of 2017.
The poll found that if Mr Cameron was able to secure a deal which "redefined the terms of Britain's membership" then 42% would definitely or probably vote to remain in the EU, with 36% saying they would probably or definitely vote to sever ties with Brussels.
But just 18% of those polled believed it was quite likely or very likely that Mr Cameron would be able to secure "satisfactory" terms, with 55% saying it was quite or very unlikely.
Opinium carried out 1,946 online interviews from June 17 to 19. Data were weighted to ensure a nationally representative sample.Suggest a correction