The King understood the 'high stakes' nature of his charm-offensive visit to Washington - but was determined to challenge Donald Trump on issues such as Nato and Ukraine 'because he personally cares'.

Speaking at the conclusion of the historic four-day trip, a senior aide also insisted that Charles was there to complement - not challenge - the government, whose relationship with Trump has been at rock bottom.

And they revealed the King's meetings with the mercurial US leader had been characterised, remarkably, by a lot of laughter behind the scenes.

The monarch's trip has widely been lauded as a triumph in helping to revive the so-called special relationship, while gently delivering some home truths to Trump on contentious issues. 'The word historic is much overused and we always hesitate to use it in the palace but I think one could use that term for this visit, ' the aide said. 'It was a high-stakes address to Congress.'

Charles was the first British king to be invited to address a joint session of the legislature, and only the second monarch after his mother, and he received no less than 13 standing ovations.

Asked why the King took the bull by the horns and raised issues including the role of Nato at a time when Trump has showed nothing but contempt for the alliance, the source said: 'It's a measure of how much he personally cares.'

As for the wisdom of addressing such hot topics, the aide added: 'First, what the King says will always be guided by the truth. Two, it will be guided by conscience. And three, they're all observable facts. Everything that is in that speech is an observable fact. So, I think we felt very comfortable with the King speaking as he did.'

They also insisted it wasn't awkward that Trump had a better relationship the monarch than he has with Sir Keir Starmer, who the President has repeatedly berated. 'It's not a competition between the King and the government,' they said. 'The King is there to support the government.'

During his visit, Charles was lauded by Trump as a 'great king' - and at times the pair were apparently seen howling with laughter together. The aide said both men - and the Queen and First Lady Melania - all 'get on very well'.

'Given some of the issues that have presented themselves, you'd think it might all be a bit tricky,' the aide said. 'But far from it.

'The warmth you see in public is absolutely the warmth you see in private. That meeting in the Oval Office, there was an awful lot of warmth and laughter.'

While the King has proved to be far from the 'meddling monarch' that many of his critics predicted, there is no doubt he has conducted this visit in a markedly different way from how his late mother would have approached it.

However aides say every monarch develops the role in their own style, and that is what keeps the institution 'relevant'. The King is apparently determined to continue to engage in issues in a way that the late Queen didn't and does not 'shy away from that'.

But sources insists he is well aware of his constitutional limitations and remains 'above politics'.

They believe the 'doom-mongers' who predicted the trip would be a disaster were 'looking down the wrong end of the telescope' and it was, in fact, an 'incredible opportunity' to reap dividends for the UK.

Trump has already announced an end to whisky tariffs 'in honour' of his 'friend', but is a notoriously volatile character.

However sources insist 'even a marginal gain, is a gain'.

It was certainly clear that Trump wanted the trip to be a success, rolling out the red carpet in a series of historic firsts. 

First Lady Melania was apparently in the White House dining room until late in the afternoon ahead of Tuesday's state dinner to ensure 'everything was exactly how she wanted it' after being blown away by the state banquet held in their honour at Windsor Castle last year.

Mr Trump even laid on a 100-strong military band to fill the room after dinner to play a series of toe-tapping Andrew Lloyd Webber showtunes, following what was described as a 'warm and handsome' tribute to the King.

Warren Stephens, US ambassador to the UK, described the state visit as 'magnificent'.

Speaking in Bermuda on Friday, shortly after the King landed for the second part of his trip - now without Camilla - the diplomat said: 'I was just telling him what a great job he did in Congress and at the state dinner and the gift of the bell of the HMS Trump.'

The artefact, taken from a Second World War submarine, was a personal gift from the King - and was a big hit with the President.

Sources say it is a sign of the King's work ethic that within seconds of taking off from the US on his government plane - 'the Baby Voyager' - on Thursday night, he had changed into his 'Bermudan attire' and got his red boxes out to read, working the whole journey.

'This is a man who is absolutely driven by duty,' an aide said. 'What looked like risk and challenge [the US] was also a phenomenal opportunity. One that was grasped in both hands by the King. We shall leave you to conclude what the outcome of that was.

'But he's not a man to dwell long on yesterday's successes, he is always looking towards tomorrow's opportunities.'