Shortly before boarding Marine One in the US, Mr Trump was asked if he planned to meet Boris Johnson during the trip.

"Well, I think I may meet with him. He's been a friend of mine. He's been very nice. I have a very good relationship with him," the president said of the Tory leadership hopeful.

He went on: "I have a very good relationship with Nigel Farage, with many people over there (in the UK) and we'll see what happens. I may meet with him. They want to meet. We'll see what happens."

Mr Farage said over the weekend he had been "banned" from meeting Mr Trump during his visit.

Accompanied by first lady Melania Trump, the president arrived aboard Air Force One on Monday morning having already created a considerable degree of political turbulence with comments on the Tory leadership race, Brexit and the Duchess of Sussex.

Ahead of the visit, he called on Britain to leave the European Union without a deal if Brussels refuses to meet its demands and urged the Government to send Mr Farage into the negotiations.

Addressing the prospect of a transatlantic trade deal, Mr Trump said: "(We're) going to the UK. I think it'll be very important. It certainly will be very interesting. There's a lot going on in the UK. And I'm sure it's going to work out very well for them.

"As you know, they want to do trade with the United States, and I think there's an opportunity for a very big trade deal at some point in the near future. And we'll see how that works out."

Anti-Trump protesters are expected to gather in large numbers in London, but Mr Trump will be largely shielded from them, as his official welcome will take place at Buckingham Palace, away from the public gaze. He will not take part in a carriage procession during his stay.

The president departed from Joint Base Andrews near Washington DC aboard Air Force One at around 8.45pm local time (1.45am BST).

The Trumps are due to arrive aboard at Stansted Airport before heading to Buckingham Palace for a reception with the Queen, Prince of Wales and the Duchess of Cornwall.

The Duke of Sussex will also attend a private lunch at the palace - although his wife, who is on maternity leave, will be at home with their four-week-old son, Archie.

Mr Trump has already denied calling the Duchess of Sussex "nasty" when he was recently confronted with comments she made before the 2016 US elections saying she would leave the country if he won.

Asked at the White House on Sunday evening if he was willing to apologise to the Royal Family, or to clarify the comments made to The Sun's political editor, Mr Trump replied: "No, I made no bad comment. Thank you."

On Sunday, Mr Khan described the president as "just one of the most egregious examples of a growing global threat" and compared the language he has used to that of the "fascists of the 20th century".

Asked if he would be willing to meet the London mayor, Mr Trump replied: "No, I don't think much of him. I think that he's a - he's the twin of (New York City mayor Bill) de Blasio, except shorter."