Washington (dpa) - US President Donald Trump said on Friday that he was working on a "very substantial" trade agreement with Britain's new prime minister, Boris Johnson.

"We are working already on a trade agreement," Trump told journalists in the Oval Office after speaking with Johnson on the phone. "I think it will be a very substantial trade agreement."

Trump claimed that the two countries were "impeded on trade" by Britain's relationship with the European Union, adding that "we can do much much more trade."

"The leaders both expressed their commitment to delivering an ambitious free trade agreement and to starting negotiations as soon as possible after the UK leaves the EU," a spokesman for the prime minister was quoted by the Press Association as saying.

Trump, who called the British leader "a friend of mine," said that during a phone call he also congratulated Johnson on his new position. "I predict he will be a great prime minister," Trump said.

Britain will be able to negotiate trade agreements with non-EU nations after it leaves the European Union.

Johnson has said a post-Brexit trade deal with the United States would be an early priority for him as prime minister.

He took over from Theresa May, who resigned after failing to win parliamentary approval for the Brexit deal she agreed with Brussels.