Donald Trump has departed the US bound for Britain aboard Air Force One.
The president, accompanied by first lady Melania Trump and an entourage of senior advisers, took off from Joint Base Andrews near Washington DC at around 8.45pm local time (1.45am BST).
He is due to arrive at Stansted Airport on Monday morning.
Mr Trump said his visit to the UK will be "very important" and spoke of "an opportunity for a very big trade deal at some point in the near future" before he left Washington.
"There's a lot going on in the UK and I'm sure it's going to work out very well for them," he told reporters at the White House.
"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."
On potentially meeting Mr Johnson, Mr Trump said: "Well 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 added: "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."
On London mayor Sadiq Khan, Mr Trump said: "No I don't think much of him, I think that he's the twin of (New York City mayor Bill) de Blasio except shorter."
Mr Trump said he made "no bad comment" when asked about an interview in which he suggested the Duchess of Sussex was "nasty".
Asked about the interview, Mr Trump replied: "No, I made no bad comment. Thank you."
During an interview with The Sun, the president was confronted with comments Meghan made before the 2016 US election saying she would leave the country if he won.
Mr Trump told the paper's political editor "I didn't know that she was nasty" in the taped interview.