US President Donald Trump is a threat to the European Union, the European Parliament's chief Brexit negotiator has said.

Guy Verhofstadt made the comment at a speech in London on Monday afternoon, warning that the union was facing a three-pronged attack from outside forces.

Two of the forces were Russia's Vladimir Putin and radical Islamism, he said, which were trying to undermine the EU project. But he added: “I have just come back from US and my view is that we have a third front that is undermining the EU ... and that is Donald Trump.”

Mr Verhofstadt, a who is a Belgian liberal MEP, warned that nationalism was no way to run Europe and that it would lead to disaster.

“Twenty million people have died because of nationalism in Europe,” he told his audience at Chatham House.

“There is not one family living on the continent and certainly not in Britain who has no grandfather, grandmother, who was not a victim of these stupidities, these atrocities at the end of the 19th and the whole 20th century.

“Putting your future organisation of Europe on nationalist ideas is the most stupid thing you can do. It’s playing with fire knowing what it has created in the past.”

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He further suggested that nationalism was in danger of making a comeback on both sides of the Atlantic.

The senior MEP was speaking just ahead of the French presidential election in which the Front National candidate is expected to do well.