And a trade deal will be a long, Herculean task’

cess, and not to implement the results of a deal, as Mrs May hopes.

“The negotiations will be large and difficult,” said Mr Van Rompuy.

“No-one knows how long, but two years will not nearly be enough.

“The length of the negotiations itself can undermine trust and push some to irrational positions. We need serenity or else the process can derail. There is no such thing as a soft negotiation, certainly not on a Hard Brexit.”

And he predicted that the UK will not be able to forge its own bilateral trade deals until after the EU FTA is sealed, as other countries will wait to see what Britain’s future relations with Europe will be before committing themselves.

“The 18 months after the triggering of Article 50 will be mainly devoted to the separation treaty,” said Mr Van Rompuy.

“You can have some more informal talks about outlines of an FTA but real sectoral negotiations will only start after the UK has left. Real negotiation with non-EU countries can only start once the FTA between the EU and UK is concluded. No country will really engage with the UK without an FTA between the EU and UK.”

Mr Van Rompuy, who was the Council’s first president, serving from 2009 until 2014, warned that crashing out of the EU without a trade deal would be “a real catastrophe for Britain and all its partners”.

But even a successful FTA will be “less advantageous than full access to the single market”, he said, adding: “Brexit is going against ... the economic interests of the UK”.

He called for a new “friendship agreement” between the EU and a postBrexit UK, under which leaders would meet regularly and co-operate closely on issues like foreign policy, defence and migration.

The UK should forge a “special relationship” with the EU of 27 states, which will be a “more stable interlocutor” for Britain than a USA led by an “erratic commander-in-chief ” at the head of the world’s largest armed forces, he said.

Britain must recognise that Brexit is not a high priority in the other 27 states, whose leaders now see the unity of the EU as their top priority in the face of the election of Donald Trump and the rise of populist, protectionist political movements across Europe.

The EU was facing the fact that it is “no longer an irreversible project”, he said.

But he voiced optimism that the forces of populism will be turned back at the ballot box, predicting the defeat of the National Front’s Marine le Pen in the French presidential race and the election of a pro-EU government in Germany. Public opinion had shifted in favour of the EU in most European countries following last year’s Brexit referendum, he said.