A dispute between Donald Trump and Justin Trudeau continued Wednesday evening when the President's son weighed in on the two leaders' diplomatic spat.

Donald Trump Jnr took a swipe at the Canadian prime minister on Twitter in a public show of support for his father, posting a now-infamous picture of Trudeau wearing blackface.

Alongside the photo, he wrote: 'As usual @RealDonaldTrump is 100% right!!! Trump calls Trudeau 'two-faced' see evidence below.'

The post comes after Trump dramatically departed yesterday to Washington from the 70th anniversary NATO summit in Hertfordshire after Boris Johnson, Emmanuel Macron and Trudeau were caught on a hot mic mocking him during a leaders' reception at Buckingham Palace on Tuesday evening.

Trump Jr. took a swipe at the Canadian prime minister on Twitter in a public show of support for his father, posting a now-infamous picture of Trudeau wearing blackface

Donald Trump Jr. (pictured last month) took a swipe at the Canadian prime minister on Twitter, writing: 'As usual @RealDonaldTrump is 100% right!!! Trump calls Trudeau 'two-faced' see evidence below'

Canadian prime minister Justin Trudeau caught in an unguarded exchange apparently gossiping about US President Donald Trump's behavior during the Buckingham Palace reception Tuesday evening

Trump and Trudeau make small talk during the NATO leaders plenary session in Hertfordshire on Wednesday morning

The president responded by calling Trudeau 'two-faced', in a clear reference to photos that emerged of Trudeau wearing black-face to parties in his 20s.

'He's two-faced... I find him to be a nice guy, but the truth is I called him out on the fact that he's not paying 2% [of national GDP towards defense] and I guess he's not very happy about it,' Trump told reporters yesterday.

'He's not paying 2% and he should be paying 2%. Canada - they have money. Look, I'm representing the US and he should be paying more than he's paying, he understands it... I can imagine he's not that happy but that's the way it is.'

But he was then later was caught on another hot mic of his own bragging about his put-down of Trudeau. 'That was funny when I said the guy's two-faced,' he was heard saying.

Justin Trudeau, center, Emmanuel Macron, second right, and Boris Johnson, right, were all caught on a hot mic at the Buckingham Palace NATO reception appearing to gossip about Donald Trump and an apparent impromptu lengthy press conference

In a clip being widely shared online the three world leaders seem to be discussing why the French President was late during the London summit. Princess Anne also appears to be in the discussion, left, but it is unclear if or how she responds

Trump then tweeted about an hour later that he would leave the summit without holding a scheduled press conference, bringing an acrimonious end to the gathering where the president made news for fighting with his fellow world leaders.

In his wake, Macron, Johnson and Trudeau scrambled to wriggle out of their embarrassing diplomatic blunder where they mocked Trump behind his back.

Macron raged that the video - said to be captured by freelance British cameraman Andy Laurence and handed to broadcasters by NATO - had surfaced at all and refused to be pressed on the gossip which was caught on hot-mic.

The French premier said: 'I am not going to comment on stolen videos. That video wasn't supposed to be filmed in that room.'

Relations between Macron and Trump were already prickly after the French president branded NATO 'brain dead', comments the US leader slammed as 'very nasty'.

The British prime minister pleaded ignorance when grilled by reporters about his part in the Palace gossip circle and said: 'I really don't know what is being referred to there.'

And his Canadian counterpart sought to squirm his way out of the awkwardness by insisting that they were not ridiculing the President but merely expressing surprise he chose to announce details of an upcoming G7 summit.

Trudeau said: 'We were all surprised and I think pleased to learn that the next G7 will be at Camp David. I think that was an unscheduled announcement and I think everyone's team, every different leader has teams that every now and then have their jaw drop at unscheduled surprises like that video itself for example.'

The Canadian PM had a quiet word and a handshake with Trump as he arrived at the summit Wednesday, and later tried to shrug off the episode.

At a press conference he said: 'As you all know, we have a very good and constructive relationship between me and the President.'

Trump looks serious as he is approached by Justin Trudeau at the summit on Wednesday, while his Canadian and UK counterparts look more light-hearted

But Trudeau wouldn't answer whether he apologized to the president for his remarks.

He said: 'Last night I made a reference to the fact that there was an unscheduled press conference before my meeting with President Trump and I was happy to take part of it. But it was certainly notable. I've had a number of a good conversations with the president over today and yesterday.'

Asked if the gaffe had given him pause for thought, Trudeau said that ensuring the focus of attention remained on matters of substance 'is something that we're all going to try to do a little harder.'

The episode began with an extraordinary hot mic moment at Buckingham Palace.

In footage from the reception, which appears to be edited, Trudeau and the other leaders are seen chuckling about Trump while Princess Anne watches on.

It shows PM Boris Johnson ask: 'Is that why you were late?' as he stoops down and chuckles in disbelief, before Canadian leader Trudeau, swigging from his drink, shoots back: 'He was late because he takes a 40 minute press conference off the top.'

The video then cuts to a later clip, which is without context, and sees Macron pointing furiously before Danish Prime Minister Rutte exclaims 'fake news media' and Trudeau adds: 'You just watched his team's jaws drop to the floor.' He then motions as if his jaw is dropping.