Johnson met Trump away from the cameras on Tuesday before a two-day NATO summit, avoiding the media fanfare that normally accompanies such talks.

He also failed to publicly greet Trump and his wife Melania on the doorstep of Downing Street before a reception with other NATO leaders.

Johnson's team are wary of what Trump might say during his two-day visit, with Britain set to go to the polls on December 12.

Johnson's Conservatives are leading opinion polls but the rival Labour party has sought to whip up public opinion against Trump, who is deeply unpopular in Britain.

In a press conference on Tuesday, the president promised to "stay out" of the election campaign and most of his remarks were about divisions within NATO.

But he still managed to endorse Johnson as "very capable" and intervene in a row over US interests in Britain's state-run National Health Service (NHS) after Brexit.

Arriving for the formal NATO talks in Watford, north of London, on Wednesday, Johnson denied he was trying to avoid being seen with Trump.

"I'm going to be photographed with every possible leader," he insisted, and later posed with the president for an official welcome alongside NATO chief Jens Stoltenberg.

Downing Street said Trump and Johnson had discussed the NATO summit and an earlier meeting between Britain, France, Germany and Turkey about the situation in Syria.

Trump wrote on Twitter that he "enjoyed" the talks, which he said covered "numerous subjects including NATO and trade".

Johnson is campaigning for re-election on a promise to get Britain out of the European Union next month, more than three years after the 2016 Brexit referendum.

He has held up a new US trade deal as one of the prizes of Brexit but has faced accusations from Labour that this will open up the much loved NHS to US firms.

Johnson denies this, and Trump said his country would not be interested in the NHS "if you handed it to us on a silver platter".

- Candid camera -

Officials are braced for Trump to become more loquacious in a press conference after the NATO talks later on Wednesday -- particularly after footage emerged apparently showing other leaders apparently laughing at him.

At a NATO reception at Buckingham Palace on Tuesday, Johnson can be heard asking French President Emmanuel Macron why he was late.

Canadian Prime Minister Justin Trudeau noted that Trump's press conference had run on, as he commented on everything from "nasty" Macron's criticism of NATO as brain dead, to "delinquent" European countries that failed to meet defence spending targets.

"Oh, yeah, yeah, he announced..." an amused Trudeau declares in the footage, adding: "You just watched his team's jaw drop to the floor.

The footage was shot by the British host's camera pool on Tuesday evening, then spotted and subtitled by Canada's CBC.