When Europe's leaders gather at a Flanders field next Thursday to commemorate the mass slaughter of a generation of young European men in the first world war, they will want to avoid another battle, particularly one between Britain and Germany.

But the EU summit in Ypres will also struggle to strike an armistice, with David Cameron and Angela Merkel, who are locking horns over the vexed question of Jean-Claude Juncker.

The former Luxembourg prime minister has been a less-than-colourful fixture of European politics for more than 20 years, the embodiment of consensus, compromise and coalition-building that is the daily grind of EU deal-making.

Suddenly and surprisingly, at what many thought was the end of a long career at the heart of EU politics, Juncker has become the most divisive figure in Europe. Cameron calls him unacceptable and illegitimate, haughtily scorning Juncker's drive to become the next head of the EU executive in Brussels. Merkel calls him a great European whom she fully supports for the job of next president of the European commission.

Senior diplomats in Brussels say "the die is cast", Merkel has won, Cameron has lost, Juncker's comeback is a fait accompli. It ain't necessarily so.

No commission chief has ever been appointed against the express and robust opposition of a big EU member state like Britain. But there's a first time for everything, and Ypres is a battlefield.

Juncker's office in Brussels is one of the few smoke-filled rooms left in the EU capital. He prefers it that way, as in his study in a modest bungalow in Luxembourg surrounded by voluminous files detailing the post-cold war history of the EU.

When he stood down as Europe's longest-serving prime minister last December in the wake of an intelligence scandal involving Britain's MI6, most thought they had seen the end of Juncker. The Europe fanatic with an encyclopaedic knowledge of the last quarter-century of EU politics was supposed to settle down to memoir-scribbling. Instead, a politician who shuns the spotlight and famously quipped he prefers "dark, secret rooms" will occupy the centre-stage in Ypres and in Brussels the following day.

For the Sun, Juncker is "the most dangerous man in Europe", the son of a "Nazi" – an improbable calumny. For Viviane Reding, a vice-president of the European commission and a Luxembourger who goes back decades with Juncker, the British campaign is nothing short of disgraceful.

"It's a shame, disgusting, low-level, has nothing to do with reality," she told the Guardian. "I'm ashamed and so sorry for what I have seen in the British press."

Juncker, too, of course, has a different story to tell.

"The loveliest European experience I've had was outside the EU, in Ukraine," he told the Guardian in April. "I organised the first summit between the EU and Ukraine. In 1997. In Kiev I took a boat trip on the river Dniepr and asked my Ukrainian colleagues how long it took to get to Odessa. They asked why. Because my father was then a forced German soldier and was wounded in Odessa. The Ukrainian asked me when that was then we cried, because his father was wounded on the same day on the Russian side.

"We tried to bring Europe forward and that's why I still believe in Europe."

The working-class son of a Luxembourg steelworker who studied law in Belgium and France has dominated politics in the Grand Duchy for a generation, as prime minister for 19 years and much of that time also combining the job with that of finance minister. Juncker is a politics junkie. He is not known to have other interests outside politics. He is married, but has no children.

Punching above his weight is an understatement when it comes to Juncker's role in what the EU has become. There are scores of city mayors across Europe running municipalities much bigger than Luxembourg's 539,000 population. But it is behind the scenes in the endless councils, conferences and summits of the EU that Juncker has left his mark. He was attending, and even chairing, meetings of EU ministers as far back as the mid-1980s. Until he quit in December, there was no one at the EU summit table bar Juncker who took part in the Maastricht summit of 1992 that created the euro.

The only politician still in office who can match his involvement in EU politics is Wolfgang Schäuble, the German finance minister, who is 12 years older.

It is the single currency, above all, that bears Juncker's imprint. The euro rule book was a mega-battle between Germany and France. It is called the Stability and Growth Pact, stability representing the German fetish, growth describing the French fixation. Juncker drafted much of it, brokering deals and bridging what appeared to be irreconcilable differences between Helmut Kohl and François Mitterrand, between the German and French finance and central bank mandarins.

When the currency Juncker helped to create started to unravel in 2010 under the weight of a ballooning banking and sovereign debt emergency, he played a key role, for better or for worse, in the crisis management as president for eight years of the Eurogroup, the committee of eurozone finance ministers that created the instruments to save the currency and plotted a wayward route out of disaster.

In the process, he bragged that you "have to lie" to succeed in such crisis management. At the height of the Greek calamity in 2011, he hosted a secret meeting of top eurozone leaders. His spokesman flatly denied the meeting took place.

When he stood down as Eurogroup president last year, his successor, Jeroen Dijsselbloem, the Dutch finance minister, criticised Juncker's handling of the sessions, pointing to his smoking and drinking habits. Juncker denied he had an alcohol problem.

Although he argues for sound public finances, supporting German-style fiscal and budgetary rigour, Juncker is often on the left "social" fringe of European Christian democracy – the centre-right led by Merkel's CDU and long the dominant force in European politics.

He espouses minimum wages across the EU, varied according to country and economic health. He supports a "European" element in national unemployment benefits schemes, aimed at softening the impact of mass unemployment in large parts of particularly southern Europe and especially among the young.

In the case of eurozone bailouts for struggling single-currency countries, he argues for coupling the stringent austerity and spending cuts packages needed to qualify for the rescue with impact assessments on the social costs of the programmes.

Under his long stewardship, Luxembourg became Europe's wealthiest country, one of the richest in the world in per capita terms, as well as a bastion of banking secrecy and a tax haven for multinationals keen to exploit the EU's single market while minimising their contributions to the European exchequer.

"He is a well-recognised statesman, a safe pair of hands," said Reding. "I have known this guy a long time. We were politicians together in the 1970s. This is a man who takes political risks, who makes realities. He is in a long line of people who have been building Europe."

Juncker is no rightwinger. He is, however, a European federalist, one reason the Conservatives maintain he is unacceptable as commission president.

This is a bit odd since, in the continental context, Juncker is in step with rather than out of kilter with the history and the tradition. All European commission presidents have been federalists to greater or lesser degrees. Whoever gets the post in the end is likely to be, too. It goes with the job. Cameron's campaign against the EU's treaty-defined "ever-closer union" is not an obsession shared with the other 27 member states.

Besides, every time Cameron or George Osborne deliver lectures about the need to reform the EU, they insist that the eurozone will need to federalise in terms of political, economic, and fiscal integration if the currency is to recover and succeed.

"Is Juncker more federalist than the other names in the frame? No," said a senior EU official. "Who is more federalist than not? If a main priority in the years ahead is governance of the eurozone, then that needs a bit more federalising."

The main objection to Juncker is less his politics and positions than the simple fact that he has been sitting around European council tables longer than anyone else, that it's time for a change, that he is a warhorse of the status quo, that last month's European parliament elections delivered a massive vote of no-confidence in the status quo and the austerity policies Juncker helped to orchestrate.

For Cameron, as well as for many other EU national leaders, the problem is less Juncker than the way he has become frontrunner for the commission job through a power play by the European parliament which, if successful, quite radically changes how the EU is run and where power lies.

Cameron, and others less combatively, complain that Juncker is being imposed by a parliamentary coup, usurping the prerogative of national leaders to agree on the nomination. It has always been the government chiefs who agreed among themselves, and always by consensus, on who should lead the commission. This time, for the first time, the party groupings in the parliament fought their election campaign with commission chief candidates leading their lists. Juncker headed the list of the European People's party, grouping the EU's Christian Democrats, which won the election.

"If Juncker wins, it will be a victory for the European parliament," said the senior official. "That makes that a federalist turning point."

Strangely, though, Juncker did not run for a parliamentary seat.

Principle and power struggles aside, there is also personal animus. Juncker, who has a sharp tongue and is heavy on irony and self-deprecation, has clashed repeatedly with Britain over the years, rallying opposition to Tony Blair in 2009 when the British politician was mooted as the first president of the European council and also clashing with Blair in 2005 in EU budget negotiations and being sour about Blair's turn at running the rotating six-month EU presidency.

But of the candidates for the job in the election campaign, Juncker was the only one making real overtures to the British. Unveiling his five-point election manifesto in April, his final point was to solve "the British question" by negotiating with Cameron ahead of the in-out referendum in the UK slated for 2017.

But Juncker told the Guardian there had been no contact with the UK government on the matter, a silence that has continued in recent weeks. Juncker and his entourage have been telephoning the British mission to the EU in Brussels pleading for an audience. The UK ambassador, Ivan Rogers, has refused to take his calls. The British insist the only right place for Cameron to discuss this is with other national leaders at the summit table. In Ypres.

"The irony is that around the [summit] table you don't find anyone who's enthusiastic about him," said a senior UK government official. "No government is coming and saying he's the best man for this job at this particular time. This is all happening in a fit of absence of mind. The implications of this have not been thought through."

That view may be widely, if quietly, shared. But it also sounds like an exasperated admission of defeat, that the momentum behind Juncker has acquired an air of inevitability, that the man voted out of office in a small country in December is about to bounce back to one of the most powerful positions in Europe.

It is unlikely Cameron can stop him. Merkel could. As could Herman Van Rompuy, chairing the Ypres summit and trying to solve one of the trickiest puzzles the Belgian has ever grappled with. Failure could be costly, plunging Europe into bitter recrimination and trench warfare just when it needs it least.

Van Rompuy and Juncker are the EU's brace of archetypal fixers, the quiet, clever men in suits who spend their lives mediating between Paris and Berlin, between big countries and small countries.

They are also rivals. There is no love lost between them, although they are of the same political party persuasion and have similar federalist views.

"Van Rompuy will bring to the table what he knows the market can bear," said the senior official. "There's too much symbolism at Ypres. We don't want a fight to the death."

Merkel's olive branch

The German chancellor, Angela Merkel, reiterated her support for Jean-Claude Juncker as the European commission's next head on Thursday, but indicated a willingness to consider concessions to Britain, which opposes him. At a press conference with the Danish prime minister, Helle Thorning-Schmidt, Merkel announced that Germany supported the former Luxembourg prime minister for the job. She also insisted on the need to take all decisions "in the European spirit".

"That implies that we take every member state seriously. That doesn't mean that one can fulfil all wishes, but it means that elsewhere one perhaps can think about what is very important for Britain," Merkel said. She added that she was ready to talk "very constructively" with Britain about issues such as reducing bureaucracy and what issues should be decided by Europe or by nation states.

The Danish prime minister told reporters she supported the candidate who could gather "the broadest possible support" in the commission and European parliament. But she said it was more important to focus on what the EU would seek to do over the next five years. AFP