US president Donald Trump has said he and Prime Minister Boris Johnson are already working on a “very substantial” post-Brexit trade agreement.

The two world leaders spoke on the telephone on Friday evening and discussed the “unparalleled” trade opportunities offered by Britain’s departure from the European Union.

Mr Trump said any deal could signal a “three to four or five times” increase in trade between the two countries.

Donald TrumpDonald Trump spoke to reporters in the Oval Office of the White House about a post-Brexit trade agreement with the UK (AP Photo/Carolyn Kaster)

“We’re working already on a trade agreement,” he told reporters at the White House. “And I think it’ll be a very substantial trade agreement.

“You know we can do with the UK, we can do three to four times, we were actually impeded by their relationship with the European Union. We were very much impeded on trade.

“And I think we can do three to four or five times what we’re doing.

“We don’t do the kind of trade we could do with what some people say is Great Britain.

“And some people, remember a word you don’t hear too much is the word England, which is a piece of it.

“But with the UK we could do much, much more trade and we expect to do that.”

Downing Street said the two leaders had “expressed their commitment to delivering an ambitious free trade agreement”.

“They agreed that Brexit offers an unparalleled opportunity to strengthen the economic partnership between the UK and United States,” a spokesman added.

Boris JohnsonDonald Trump has said Boris Johnson is going to be a ‘great Prime Minister’ (Jacob King/PA)

Mr Trump said the UK had “needed” Mr Johnson for a long time, adding: “He has what it takes.”

“I think we can have a great relationship and Boris is going to be a great prime minister,” he said. “I predict he will be a great prime minister.”

The president had previously heaped praise on Mr Johnson following his victory in the Tory leadership contest.

Addressing a crowd of young US conservatives on Tuesday, he said: “We have a really good man who is going to be the prime minister of the UK now: Boris Johnson.

“Good man. He’s tough and he’s smart. They’re saying ‘Britain Trump’. They call him Britain Trump and it’s people saying that’s a good thing.

“They like me over there. That’s what they wanted. That’s what they need.”

The leaders also used their Friday night call to discuss tensions with Iran.