Donald Trump has said that the UK is in “turmoil” and it is “up to the people” whether or not Theresa May remains Prime Minister.

Speaking as he prepared to depart for a European trip in which he will meet the Prime Minister at the Nato summit in Brussels and then make his first visit to the UK as US president, Mr Trump insisted he got along with Mrs May “very well”.

But he suggested that he might take time during his UK visit to speak with former foreign secretary Boris Johnson, who he described as “a friend of mine”.

Donald TrumpMr Trump said he got on very well with former foreign secretary Boris Johnson (AP)

Answering questions from reporters on the lawn of the White House as he and his wife Melania prepared to board the Marine One presidential helicopter, Mr Trump said that the UK “certainly has a lot of things going on”.

“It’s going to be an interesting time in the UK and an interesting time at Nato,” he said. “We will work it out and all countries will be happy.

“So I have Nato, I have the UK – which is in somewhat turmoil.”

Referring to Monday’s planned summit with Russian president Vladimir Putin in Helsinki, Mr Trump added: “And I have Putin. Frankly, Putin may be the easiest of all.”

Asked whether Mrs May should be replaced as Prime Minister, the US president replied: “Well that’s up to the people. I get along with her very well, I have a very good relationship. That’s certainly up to the people, not up to me.”

Mr Trump said he had not spoken with Mrs May, adding: “Boris Johnson is a friend of mine. He’s been very, very nice to me, very supportive. Maybe I’ll speak to him when I get over there.

“I like Boris Johnson, I’ve always liked him.”

TrumpFirst lady Melania Trump and President Donald Trump board Marine One on the South Lawn of the White House (AP Photo/Evan Vucci)

Mr Trump repeated his demands for the EU to lower barriers to US imports and for America’s Nato allies to pay more towards their own defence.

“We do have a lot of allies, but we can’t be taken advantage of,” he said. “We are being taken advantage of by the EU.

“We lost 151 billion (US) dollars last year on trade and on top of that, we spend at least 70% to Nato and frankly it helps them a lot more than it helps us.

“So we will see what happens, we have a long, beautiful week.”