David Cameron faces humiliation at a two-day European summit opening on Thursday in his quixotic campaign to stop Jean-Claude Juncker becoming the next head of the EU executive.

Berlin brushed aside his opposition to Juncker on Wednesday, acceded to British insistence on a summit vote on the issue on Friday and announced that it had a "very big majority" in its support.

As British officials recriminated over what they see as treachery by Angela Merkel, who had previously assured Cameron of her reservations over Juncker, a senior German official dismissed the UK tactics.

"As I understand it, it is the British who are pushing for a vote," he said. "I assume that the nomination of Herr Juncker will find a very big majority."

At a two-day EU summit in Ypres in Flanders and in Brussels opening on Thursday, Cameron is to demand a vote among the 28 EU national leaders if Juncker is nominated as next president of the European commission. Such decisions have always been taken by consensus, impossible without British assent.

Merkel was withering about the British tactics. "We have to have a majority vote. It's not a drama if we decide by qualified majorities only," she told parliament in Berlin. "Germany supports Jean-Claude Juncker."

The British government is resigned to the lost cause that has been Cameron's aggressive, personalised battle to halt Juncker, the former Luxembourg prime minister.

Following talks on Monday in London between Cameron and Herman Van Rompuy, the president of the European council chairing the two-day summit and charged with mediation on the Juncker issue, it is virtually certain that the Belgian will formally nominate Juncker, overriding British objections.

"It's clear we will end up with a vote," admitted a senior British official. "There will be a decision."

Amid a mounting sense of crisis for Britain in Europe, the UK ambassador to the EU, Ivan Rogers, was summoned back to Downing Street on Wednesday for last-ditch consultations before the summit kicks off on Thursday with first world war commemorations at the Ypres war graves complex west of Brussels.

Cameron spoke by phone to Merkel and the Dutch prime minister, Mark Rutte. "The prime minister explained that his opposition would not change. Chancellor Merkel and Prime Minister Rutte recognised the PM's position and agreed that if the [summit] decides not to proceed by consensus then there should be a vote," said a Downing Street spokesperson.

All the signs were that the Juncker showdown and Cameron defeat may mark a caesura in the UK's vexed relationship with and membership of the EU.

The Cameron government is said to have been exhaustively exploring all its options, including invoking an obscure mechanism from the 1960s citing "vital national interests" to automatically halt a decision at a summit, a move that looks legally dubious.

The option was sent to the attorney general for legal review. Senior officials and diplomats in Brussels said resorting to the so-called "Luxembourg compromise" would not pass legal muster.

It is not clear how the prime minister will respond following what looks like certain defeat and when he faces the House of Commons on Monday. Speculation focused on Cameron delivering an ultimatum to the rest of the EU about the need for radical reform and threatening to recommend a no vote in his mooted in-out EU referendum slated for 2017.

"This is a big moment. We have to work through the consequences of where that takes us and the EU and what we do about it," said a British official. "There will be consequences. It will be a decision by Her Majesty's government … Both the prime and deputy prime minister believe this a direction that is profoundly wrong, a huge mistake. It is a big issue and, if it happens, we do not want to minimise it. You can't say nothing much has happened here and there are no consequences.

"The prime minister has been reflecting a great deal on it, there have been a lot of discussions and in terms of his own agenda, the reforms he wants to see in the EU, this is a significant step in the wrong direction … This creates real issues for us as to the way in which the EU is run."

It is clear that Cameron and his entourage feel double-crossed by Merkel, who has blown hot and cold in her support for Juncker. Both governments have discussed the Juncker issue repeatedly over the past six months. The British were confident Merkel shared their opposition, only for her to reverse that position in the light of last month's European elections as a result of hostile domestic media coverage and infighting within her coalition of Christian and social democrats.

There were also charges that Cameron had acted ineptly with the German chancellor. The government's position and Cameron's tactics came under attack from Lord Kerr of Kinlochard, Britain's veteran former EU ambassador, who accused Cameron of having "burnt" Merkel.

Kerr told the Guardian: "Jean-Claude Juncker would have been dead if we had decided to play it correctly. We played the man not the ball, which is fair enough provided you are not caught doing it. We did it too obviously. We had no concepts. "We burnt her [Merkel] off. It became impossible for her to support us."

Kenneth Clarke also criticised the demonisation of Juncker. "He is not an arch-villain. I can think of better candidates," he told Radio 4. "No one knows what [Juncker] is supposed to have done wrong. The idea that he is an arch-federalist, a sort of public enemy number one which the media have made him in the last few days, is slightly exaggerated."