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BERLIN (Reuters) - U.S. President Donald Trump poses one of the biggest challenges for the European Union because he is “praying” for the breakup of the bloc, European Council President Donald Tusk said.

Trump has welcomed Britain’s decision to leave the EU, labeling Europe a trade “foe” and questioning the value of the transantlantic defense alliance NATO.

“For the first time in history, there is an American president who is openly against a united Europe. He supports Brexit and prays for the breakup of the Union,” Tusk told the German weekly newspaper Die Zeit in an interview.

Tusk portrayed Trump as determined to weaken the EU and said this was “perhaps the most difficult challenge” for the organization.

Tusk, a former Polish prime minister, is about to complete his five-year mandate as president of the European Council, a role in which he has prepared and chaired regular summits of European leaders.

Though ties between the EU and the United States have been troubled since Trump became president, U.S. Secretary of State Mike Pompeo sought to re-establish friendlier ties when he visited Brussels in September.

Because of its complex matrix of decision-making, multiple institutions and centers of power, the EU has 28 national leaders and institutional leaders in Brussels.

Tusk is expected to be succeeded by former Belgian Prime Minister Charles Michel. The new European Commission, the EU executive, will be headed by German conservative Ursula von der Leyen.