Trump added he thought Trudeau was a "nice guy" and was probably "not very happy" that he had been called out over not meeting Nato's 2 per cent spending thresholds.

He then cancelled a news conference scheduled for the end of the two-day summit and headed back to Washington early. Trump's team cited the extensive press statements he had already made at the event.

Had it gone ahead, Trump would have likely faced questions about a House Judiciary Committee and its impeachment hearing against him, taking place in the US at the same time.

The harsh words for Trudeau come after the Canadian Prime Minister was caught on camera

.@JustinTrudeau, @EmmanuelMacron, @BorisJohnson and other VIPs shared a few words at a Buckingham Palace reception Tuesday. No one mentions @realDonaldTrump by name, but they seem to be discussing his lengthy impromptu press conferences from earlier in the day. (Video: Host Pool) pic.twitter.com/dVgj48rpOP

— Power & Politics (@PnPCBC) December 3, 2019

 speaking to UK Prime Minister Boris Johnson, French President Emmanuel Macron, Dutch Prime Minister Mark Rutte and Britain's Princess Anne at a reception for Nato leaders at Buckingham Palace.

"Is that why you were late?" Johnson can be heard asking Macron as he wanders up to the group, in what is assumed to be a reference to a press conference the French President held with the US leader.

Trudeau interjected: "He was late because he (Trump) takes a 40-minute press conference off the top … oh yeah, yeah, yeah, he announced …"

Some of the comments are inaudible, but Trudeau can be heard saying: "You just watched his team's jaws drop to the floor."

Trump's name cannot be heard, however the comments are widely assumed to be about the US leader's extraordinary press conference on Tuesday morning UK time, where he said Macron was being "insulting" and "dangerous" by referring to the Nato alliance as "brain dead".

"Turkey responded by saying that he was brain dead which was interesting," Trump said at the time.

"I think that's very insulting to a lot of different forces. It has a great purpose."

"That's a very, very nasty statement," he continued. "[France] have a very high unemployment rate.

"You look at what's happened with the yellow vests …. they've had a very rough year. You just can't go around making statements like that about Nato, it's very disrespectful."

The pair later spoke to the press together.

On Wednesday, after the intimate conversation had been broadcast around the world, UK PM Boris Johnson also faced questions over what was discussed.

"That's complete nonsense I don't know where that's come from," he said.

"I really don't know what is being referred to there."

Johnson was also asked whether he believed President Trump was an "amnesiac or a serial liar" following Trump's claims he had "didn't know" Prince Andrew.

Johnson laughed as he praised the question as a "good effort" to drag in an unrelated issue.

"None of those subjects was mentioned in any of the sessions nor to the best of my knowledge, in any of the bilaterals that have taken place," he said.

The spat between the two North American leaders is not the first time they have disagreed in public.

Image / Twitter

After the G7 meeting in June Trump said the Canadian leader acted "so meek and mild during our G7 meetings only to give a news conference after I left saying that, "US Tariffs were kind of insulting" and he "will not be pushed around." Very dishonest & weak," he tweeted.

'Iron-clad' collective defence

The 70th anniversary has been a fractious affair for 29 members of the alliance and the royal family hosting them, which are both facing internal divisions of their own.

Nato has been struggling with a lack of strategic direction, tensions among members and the transactional foreign policy style of President Trump. The royal show of unity was notable as Prince Andrew was missing after being stood down from royal duties. Prince Harry and Meghan were also absent while on a six-week hiatus in the US.

President Trump has successfully forced other nations into boosting their share of defence spending, reducing the burden on the US, but has also pulled US troops out of northern Syria with little warning, paving the way for a Turkish invasion that shocked Nato allies.

On Wednesday, President Trump defended the alliance as having "great spirit" and said "Nato is stronger than it's ever been."

A joint leaders' declaration issued on the anniversary said "Nato remains the foundation for our collective defence and the essential forum for security consultations and decisions among allies."

Nations committed to spending at least two per cent of their budgets on defence and said more than US$130 billion more is being invested.

The group also affirmed it's commitment to collective defence saying: "our solemn commitment as enshrined in Article 5 of the Washington Treaty that an attack against one ally shall be considered an attack against us all."

Nato Secretary-General Jens Stoltenberg said Wednesday that "we stand together, all for one and one for all. Our commitment to article 5, the collective defence clause of our alliance, is iron-clad."