Donald Trump has insulted London mayor Sadiq Khan moments before setting off for Britain.

Asked on Sunday evening if he would be willing to meet Mr Khan during his state visit, Mr Trump said: "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."

It came after Mr Khan described the US president over the weekend as "just one of the most egregious examples of a growing global threat" and compared the language he had used to that of the "fascists of the 20th century".

Responding to reporters over the din of his Marine One helicopter as he left the White House, Mr Trump said he expected his three-day state visit to be "very important" and "very interesting".

"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," Mr Trump said.

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"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 Mr Johnson.

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."

Accompanied by first lady Melania Trump, the president will arrive 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.

The controversy was further exacerbated on Sunday when the US ambassador to the UK, Woody Johnson, suggested the NHS would be on the table in post-Brexit trade talks.

Sky News promo for Trump's UK state visit features baby blimp

After landing at Stansted Airport, the Trumps will head 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, Mr Trump replied: "No, I made no bad comment. Thank you."

Mr Trump's arrival in the capital is due to be followed by a protest through central London against the visit on Tuesday, with the "Trump baby" blimp made for his visit last year due to put in an appearance.