The battle for Brexit, like the Battle of Waterloo, may ultimately be won or lost on the playing fields of Eton between two famous old boys, David Cameron and Boris Johnson. But that does not mean European, and American, politicians will be entirely absent from the pitch in the next three months. Indeed, they could play a critical role both by what they say and how effectively they manage to run the European Union in the months ahead.

The question for overseas diplomats is in which role can their leaders best prevent Brexit. Should the UK’s EU partners cast themselves as concerned but mute bystanders, or instead intervene, as David Cameron did in the Scottish referendum, passionately expressing their love and begging a restless partner not to leave the marital home?

There is no doubt that EU heads of state want the UK to reject Brexit, so avoiding a political and economic crisis at a time when Europe, gripped by the migration crisis, simply cannot afford more disruption. Britain, as the world’s fifth-largest power, adds to the heft of the EU, and its pragmatism increases its diversity. Many are worried about the contagion of referenda spreading through Europe.

But the question for Downing Street is whether the British respond well to a succession of European leaders with thick accents, and variable grasp of the English language, appearing to tell them how to vote.

In a debate about national sovereignty, the voices of European elites, sometimes short of pitch perfect to the British debate, could prove counterproductive.

The European Commission has taken a virtual vow of silence and is culling any directives that could, for example, be seen as a threat to the British tea bag. Anand Menon, professor of European politics at King’s College, reports: “In Brussels, senior figures in all the institutions have been told not to denigrate Cameron’s deal, or suggest it is not binding in law. There was a bit of that after the summit, but it has now stopped. It is also inadvisable that there are set-piece speeches setting out their love for Europe”.

But British pro-Europeans such as Lord Mandelson are urging the UK’s European partners to find a balance between warning of the dangers of Brexit in terms of access to the single market and trade deals without sounding bullying. Lord Mandelson said: “The referendum is a purely British decision and no one would expect others to advise us how to vote. Our European partners want us to stay in the EU and so it would not be surprising for them to say so and explain why Europe should not be weakened by Britain’s exit.”

He added: “There are also figures in Brussels who are very much on the reform wavelength and share Britain’s economic outlook, but the difficulty is, would British newspapers report them without distorting what they say?”

Barack Obama will use a spring visit to UK to urge country not to leave EU. Photograph: Evan Vucci/AP

The Downing Street judgment is that it is unavoidable world leaders are recruited to the cause, so long as it is coordinated with Number 10. The recent explicit statement by G20 finance leaders in China that Brexit would be a threat to the world economy was inserted at the request of the chancellor, George Osborne. Barack Obama, still a trusted voice in the UK, will use his visit to Europe in the spring to reach out and urge Britain to stay in the EU. This is not just a favour to Cameron. Belatedly there is a recognition in Washington of the strains inside the EU caused by the migration crisis.

In a bid to corral the intervention, David Cameron’s Downing Street adviser Daniel Korski met senior diplomats from the 27 other EU states in London last Friday to take them through the UK government campaign, and urge them to think carefully about their role.

Korski urged diplomats that EU politicians should think twice before publicly belittling or criticising the deal achieved by Cameron in Brussels. François Hollande, the French president, was mentioned as being pretty dismissive of the deal. Korski suggested that Number 10 might be consulted prior to any speeches or other big statements even if they are aimed at domestic audiences as they may have an impact on the UK debate, according to reports.

“It is just sensible that people realise that what they say in their own countries can have an impact here – to be aware that there’s an impact in the UK. If a leader stands up and says something to their own parliament, it won’t just play in their own domestic media, it will make it to the UK.”

But some diplomats are tentatively dipping their toe in the British debate.

The German ambassador, Dr Peter Ammon, has held discreet breakfasts with British journalists to underline the importance of the UK to Germany. This week he was even on the campaign trail telling a London symposium that the choice was voting with your head or heart. “I want the Brits to keep their heads,” he said. “The romantic dream of restored sovereignty would turn out to be a pipe dream, because in a world of globalised political decision-making as well as globalised production chains and globalised financial markets, it could mean less power over your country’s fate, not more”. But Ammon was also modest enough to acknowledge the limits of his persuasive powers. “I realise most Brits do not wake up each morning asking themselves what do the Germans think.”

It is likely that there will also be a string of European finance ministers such as Italian finance minister Pier Carlo Padoan and his German counterpart Wolfgang Schaeuble travelling through London to make their warnings about Brexit.

David Cameron has provoked debate about what would happen to the Calais refugee camp if UK leaves EU. Photograph: Gareth Fuller/PA

The dilemma comes to a head in its most acute form on Thursday when David Cameron and François Hollande meet for the annual Anglo French summit in Amiens.

The Elysée is aware the number one question for the British press at the summit is whether Hollande agrees with Cameron that in the event of Brexit, the French would be inclined to tear up the bilateral treaty signed in 2003 that allowed the UK border effectively to be moved to Calais.

In short, will Hollande agree with Cameron that Brexit means the “Jungle”, or the “swarm”, moves from Calais to the Kent coast? The warning has been denounced by leading eurosceptics such as David Davis and Liam Fox as scaremongering.

Hollande could simply say, as has the French ambassador to London, that the French reaction is unknowable and instead focus on the recent measures the security forces have taken this week to reduce the camp’s size, disperse more refugees to new centres elsewhere in France, speed up the processing of asylum claims and improve security around the Channel tunnel entrance.

But Cameron, eager to highlight the threat to UK borders in the event of Brexit, will hope Hollande will stand up his warnings about the consequences of Britain leaving the EU.

Hollande is under conflicting domestic pressure. The left of his party is threatening to block him from standing in next year’s elections, fed up with his shift to the right. But Alain Juppé, the mayor of Bordeaux and the man at present most likely to win the presidential elections in June, has recently visited Calais and denounced it as unacceptable.

Juppé’s political ally in the region, Xavier Bertrand, the recently re-elected president of the Nord-Pas-de-Calais-Picardie region, has repeatedly said the Le Touquet agreement would be torn up if Britain left the EU. He said: “If Britain leaves Europe, right away the border will leave Calais and go to Dover. We will not continue to guard the border for Britain if it’s no longer in the European Union.”

Emmanuel Macron, the economy minister, in an interview with the Financial Times on Thursday, warned: “The day this relationship unravels, migrants will no longer be in Calais.”



By contrast, Hollande’s own interior minister, Bernard Cazeneuve, appeared to reject different solutions. In October, he said: “Calling for the border with the English to be opened is not a responsible solution. It would send a signal to people smugglers and would lead migrants to flow to Calais in far greater numbers – a humanitarian disaster would ensue; it is a foolhardy path and one the government will not pursue”.

So Hollande is between a rock and a hard place. Whatever he says he will offend one side or other in the Brexit debate. He is unlikely to be the last European politician to face this dilemma in the months ahead.