Downing Street was braced for fireworks with Trump arriving for a meeting of the NATO military alliance just days before the December 12 vote.

Answering questions from reporters at a meeting with NATO chief Jens Stoltenberg, Trump said he would "stay out of the election", adding: "I don't want to complicate it."

But he confirmed he would be meeting Johnson during his trip, adding: "I think Boris is very capable and I think he will do a good job."

There were no immediate details of a one-to-one meeting.

Opinion polls suggest Johnson's Conservative party is heading for a comfortable win next week but his team is nervous about any incendiary remarks from the unpredictable Trump.

The US leader has been outspoken in his support for Britain's looming exit from the European Union, which Johnson has promised to deliver in January after years of turmoil.

But he has previously questioned whether the terms Johnson has struck with Brussels would allow Britain to make a trade deal with the US.

Trump also refused on Tuesday to repeat his previous criticism of main opposition Labour leader Jeremy Corbyn, saying: "I can work with anybody, I'm a very easy person to work with."

Labour has sought to capitalise on Trump's visit by repeating its claims that Johnson would open up Britain's state-run National Health Service (NHS) as part of a US trade deal after Brexit.

Johnson has repeatedly denied this. Trump also denied he was interested.

"If you handed it to us on a silver platter, I would not take it," he said.

The prime minister is hosting the NATO summit and will attend a reception for the alliance's leaders at Buckingham Palace on Tuesday evening, hosted by Queen Elizabeth II.

Corbyn boycotted a royal dinner during Trump's state visit to Britain earlier this year but his party said he would attend Tuesday's event.