European leaders have come to a 27-nation consensus that a “hard Brexit” is likely to be the only way to see off future populist insurgencies, which could lead to the break-up of the European Union.

Why Europe will drive a hard Brexit Read more

The hardening line in EU capitals comes as Nigel Farage warns European leaders that Marine Le Pen, leader of the Front National, could deliver a political sensation bigger than Brexit and win France’s presidential election next spring – a result that would mean it was “game over” for 60 years of EU integration.

According to senior officials at the highest levels of European governments, allowing Britain favourable terms of exit could represent an existential danger to the EU, since it would encourage similar demands from other countries with significant Eurosceptic movements.

One top EU diplomat told the Observer: “If you British are not prepared to compromise on free movement, the only way to deal with Brexit is hard Brexit. Otherwise we would be seen to be giving in to a country that is leaving. That would be fatal.”

Facebook Twitter Pinterest Nigel Farage predicted Brexit and Trump’s presidential victory. Photograph: Lauren Hurley/PA

The latest intervention by Farage will only serve to fuel fears in Europe that anti-EU movements have acquired a dangerous momentum in countries such as France and the Netherlands, following the precedent set by the Brexit vote. Ukip’s interim leader, who predicted both the vote for Brexit and Donald Trump’s US victory, told the Observer that while Le Pen was still more likely to be runner-up to an establishment candidate next May, she now had to be taken seriously as a potential head of state.

“She will clearly win through to the second round. And after what has happened elsewhere, only a fool would say she would have no chance of winning overall. France is a deeply, deeply unhappy country. If she were to win, it would be game over for the EU.”

Le Pen has made clear she wants to take France out of the euro and the EU. The prospective hardline approach has been agreed by the 27 member states as a bloc. Acting in concert, the remaining EU states will refuse to grant the UK access to the single market unless London agrees to sign up to its rules, including free movement of people, capital, services and labour.

Norbert Röttgen, chairman of the foreign affairs committee in the German Bundestag, told the Observer that he wanted to help the UK but, if there were no willingness to accept EU rules, there was no hope. “I am really ready to come to a result but if [the British position is] no, no, no, then even I would have to say that there is no common ground.”

The Observer view on the worldwide threat to liberal democracy | Observer editorial Read more

The vice-president of the centre-right European People’s party, German MEP David McAllister, added: “You can’t have 100% control over internal migration, say no to European Court of Justice rulings, and that you won’t pay anything [into the budget]. This just won’t work.

“The British are testing us – we all know that. They are testing how united Europe is. So what is important is that Europe stays together. No bilateral negotiations with the British. No cherry-picking. We are doing this as a bloc.”

Trump’s sensational victory led some members of the French establishment, and politicians in Berlin and Brussels, to worry that Le Pen could benefit from a similar wave of popular discontent and pose a real threat. Former French prime minister Jean-Pierre Raffarin said reason no longer prevailed in France and that Le Pen “can win”.

Facebook Twitter Pinterest Norbert Röttgen has said he wants to help the UK but that it must accept EU rules. Photograph: Bob Strong/Reuters

Farage, who has been heavily critical of the Front National as a party, but admires Le Pen personally, said he had not had time to think about whether he would support her campaign for the Elysée. “It has been too busy,” he said. “What I would say is that I have not had anything good to say about the National Front but equally not anything bad to say about Marine Le Pen. I think the important thing for her is to run as much as possible as an individual, as her own person.”

French MEP Constance Le Grip said she did not believe Le Pen could win, but admitted there was a danger of bolstering support for her if the UK were given an exit deal that was too favourable.

“If you start to reshape the whole project to be some sort of Europe à la carte it is the beginning of the end, apart from the fact that Le Pen will then claim victory, Wilders in the Netherlands, and so on. More opt-outs for the UK would be absolutely incoherent and absurd. ”

On 29 October, less than two weeks before Trump seized the White House, Farage rejected prevailing opinion and forecast his victory against Hillary Clinton. “I am going to predicthere and now that Trump is going to win.” On 1 June – 22 days before the Brexit vote – he said: “I believe we will win.”