“So many are becoming openly anti-European”, he said, adding: “Particularly the change in Washington puts the European Union in a difficult situation; with the new administration seeming to put into question the last 70 years of American foreign policy”.

Tusk’s letter was addressed to the 27 EU leaders who will meet in Malta on Friday to discuss what the EU should do in reaction to Brexit.

It came out one day after Guy Verhofstadt, a former Belgian PM and a leading MEP, said that Trump was a "third front undermining the European Union" alongside Russia and Islamist terrorism.

The French and German leaders have also criticised Trump, with French president Francois Hollande saying prior to Trump's election that Trump made him want to vomit.

Trump, who took up office one week ago, has said he expected other member states to leave the EU and has called Nato “obsolete”.

He has also lent support to anti-EU populists such as British MEP Nigel Farage and French presidential candidate Marine Le Pen.

Tusk, a former Polish prime minister who now chairs EU summits, said the rise of “nationalist egoism” inside the EU and a “decline of faith in political integration” in what he called “the pro-European elites” also posed threats.

He said the EU should react to “words and … decisions aimed against the EU” with a “European pride”.

“We must stand up very clearly for … the dignity of a united Europe - regardless of whether we are talking to Russia, China, the US or Turkey,” he said.

He said Europe was “the best place on Earth” and that “the times of European unity have been the best times in all of Europe's centuries-long history”.

He also called it a “trade superpower”.

He warned that if the EU disbanded it “would not lead to the restoration of some mythical, full sovereignty of its member states, but to their real and factual dependence on the great superpowers: the United States, Russia and China.”

He said the Malta summit would yield “an ambitious declaration” on unity and that it would lead to “assertive and spectacular steps” on issues such as the migration crisis.

He added that one of those would be to secure Italy’s borders against migrants coming from Libya.

“Flows are at a record level, too many people die while trying to reach Europe, and spring is approaching fast,” he said.

Tusk repeated his Trump threat assessment after a meeting with the leaders of Estonia, Lithuania, and Latvia in Tallinn also on Tuesday.

The Baltic leaders did not mention Trump, but did call for Nato and “transatlantic” unity in the face of Russia’s menacing behaviour in a brief press conference.

Estonian prime minister Juri Ratas said he had spoken to Ukrainian leader Petro Poroshenko amid reports that a flare-up in fighting with Russia’s proxy forces killed eight more people this week.

Latvia’s Maris Kucinskis said Russia posed “a real challenge” and called for European countries to increase their defence spending.

Lithuanian prime minister Saulius Skvernelis called for EU states to negotiate as one with the UK on Brexit and said Baltic states should reduce their energy dependence on Russia.