Donald Trump has been overheard hailing his "funny" comment about "two-faced" Canadian Prime Minister Justin Trudeau.
At a Buckingham Palace reception for NATO leaders on Tuesday night, Mr Trudeau was caught on camera apparently gossiping about the US president.
Mr Trump gave his cutting assessment the day after his Canadian counterpart was seen talking to French president Emmanuel Macron about an unnamed associate during the reception to welcome leaders to the NATO summit in London.
The US president branded Mr Trudeau "two-faced" and suggested the Canadian prime minister was not impressed with him pointing out Canada is not paying its promised 2% of GDP on defence.
"I find him to be a very nice guy but you know the truth is that I called him out the fact that he's not paying 2% and I can see he's not very happy about it," he said.
"He's not paying 2% and he should be paying 2%. Canada - they have money."
Mr Trump was later caught in an audio recording commenting to an unidentified individual about his decision to cancel his close-of-summit news conference.
The individual said: "You'll be in double digits for press conferences."
The US president replied: "And then you know what they'll say? He didn't do a press conference! He didn't do a press conference!"
In apparent reference to Mr Trudeau, Mr Trump then added: "That was funny when I said the guy's two-faced."
Prime Minister Boris Johnson, who was standing with Mr Trudeau, Mr Macron and Princess Anne when the footage was recorded, dismissed the video as "nonsense".
He told Sky News' foreign affairs editor Deborah Haynes he "doesn't know where that's come from".
Mr Trudeau also also moved to play down the video, insisting he enjoys a "very good relationship" with Mr Trump and that the pair shared a "great meeting" during the NATO summit.
Luxembourg Prime Minister Xavier Bettel later passed on some advice, saying: "Don't speak - even when you think no-one is listening."
Ahead of the Buckingham Palace reception, Princess Anne appeared to be in trouble with the Queen who pointedly looked at her daughter while greeting Mr Trump and his wife, Melania, before the princess, who was talking to someone else, shrugged her shoulders and laughed - as the Queen looked away stony-faced.
During the ensuing reception Mr Macron, who riled Mr Trump by describing NATO as suffering from strategic "brain death" , was heard joking with Mr Trudeau, Boris Johnson and Dutch prime minister Mark Rutte about a "40-minute press conference" that had run late.
When Mr Johnson asks Mr Macron "is that why you were late?", Mr Trudeau replies "he's late because he takes a 40 minute press conference off the top" - seemingly a reference to the Macron-Trump meeting earlier in the day.
Princess Anne is also seen on the fringes of the conversation, which appeared to show Mr Trudeau making another cheeky remark about the press conference.
He tells his fellow leaders: "You just watched his team's jaws drop to the floor."
The NATO summit in London was notable for a few frosty meetings between the prime ministers and presidents who have gathered, notably between Mr Trump and Mr Macron.
Mr Trump certainly made his presence felt in London during several media briefings on Tuesday, not just in his tense appearance alongside Mr Macron.
The US president used a press conference with NATO Secretary-General Jens Stoltenberg to say he would not be offering any comment on the UK general election - only to then immediately do just that .
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Mr Trump said: "I'll stay out of the election. I'm a fan of Brexit, I called it. I was here, they asked if it was going to happen and I said yes and they smiled, it was just my opinion.
"I'll stay out of it, but Boris is very capable and I think he will do a good job."
Mr Trump and his fellow leaders capped off the day by marking the 70th anniversary of NATO at Buckingham Palace, where they posed for a traditional group photo alongside the Queen.
On Wednesday, Mr Johnson addressed leaders from all 29 member states at The Grove hotel in Hertfordshire.
Upon his arrival at the venue, Mr Macron said he did not regret saying NATO was experiencing "brain death" because it had spurred the right kind of debate about the direction of the alliance.
He told reporters that the remark "allowed us to raise some crucial debates".
Mr Johnson used a speech at the summit to attempt to bring harmony to the top of the alliance by echoing a famous phrase from the classic French adventure story The Three Musketeers, reminding colleagues that it is "one for all, and all for one" in the quest to keep their people safe.
The meeting was expected to consider new threats, including in the areas of cyber and space, after NATO last month declared they were among its operational domains alongside air, land and sea.
In his opening address, Mr Johnson said members were "rock solid in our commitment to NATO" and that leaders had the opportunity to "strengthen the unity of purpose" of the alliance.
Analysis: Airing spats out in the open plays into the hands of Russia
By Alistair Bunkall, defence and security correspondent
After 48 hours of public bickering, the NATO leaders' gathering ended with two almighty sighs of relief.
First, the final declaration was passed without dissent; and then, secondly, Donald Trump cancelled his planned news conference and decided instead to head straight back to Washington.
That either of those can be considered good outcomes tells you much about how the rest of the meeting went.
We all knew it would be tense. NATO itself kept the event short to avoid opportunity for dissent.
Even so, some leaders (one in particular) just couldn't help themselves.
Usually these spats are played out behind closed doors.
To air them in the open only plays into the hands of NATO's main enemy, Russia, which seeks to divide wherever possible.
Read full analysis here .