King Charles was determined to undertake the “high stakes” visit to the US despite criticism that it had become too political, as he grasped the opportunity to help repair the special relationship with “both hands”. He was not sent to fill the gaps where Prime Minister Keir Starmer has failed to build rapport with President Donald Trump’s administration, a senior Palace aide involved in the visit said.

But the King would never shy away from asserting his beliefs and standing up for Britain and democracy on the world stage. And nowhere was this clearer in his speech to Congress on Tuesday. He did not shy away from the tricky topics of Ukraine, the importance of NATO, the need to safeguard the environment and provide checks and balances on executive power.

It was carefully crafted, only subtly criticising President Trump’s administration and ensuring his speech was delivered with respect and spades of his characteristic humour and wit.

Reflecting on the speech, the Palace aide said: "It was a high-stakes address to Congress.” Asked why the King wanted to include the potentially thorny topics, they added: “It was a measure of how much he personally cares.”

They 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. And so, I think we felt very comfortable with the King speaking as he did."

There had been calls for the Government to call off the four-day state visit, due to the complex political backdrop to the trip, which had left the special relationship hanging by a thread.

A myriad of events threatened to put a stop to the trip, which had been in the diary for years. The Iran war, President Trump’s public fallout with the Prime Minister, his belittling comments about the British military, and then the added security complexity after gunshots were fired at a Washington dinner attended by the President and First Lady.

But the King, highly skilled in diplomacy after years of travelling the globe on behalf of the British Government, did not want to shy away from an opportunity to mend fences between the two countries.

"What looked like risk and challenge was also a phenomenal opportunity,” the senior Palace aide said. “One that was grasped in both hands by the King and we shall leave you to conclude what the outcome of that was."

Everywhere we visited following the speech, from Washington to New York and small towns in Bermuda, people volunteered their praise of the King’s highly diplomatic yet pointed address.

A poll published on Saturday reveals that the vast majority of Britons believe the King performed well in America. Three-quarters of those polled by YouGov on May 1 said he handled the visit well, with just four percent thinking he handled it badly.

And even when the President went off piste and claimed Charles “would probably have helped us with Iran”, there was no sharp intake of breath from the Palace team as they recognise that Mr Trump is unpredictable, and has a habit of saying what is on his mind.

Palace sources described the trip as “historic”, and not just because it took place on the 250th anniversary of American independence. But there’s no doubt that the King’s speech to Congress will go down in history as one of the most remarkable he has ever delivered.

He has delivered thousands of speeches, but few have fallen against such a politically fraught backdrop and yet been so widely celebrated by people from all sides of the political spectrum.

Mr Trump summed it up perfectly himself when he opened his speech at the state dinner to heap praise on the King’s address. “I want to congratulate Charles on having made a fantastic speech today at Congress,” Trump said. “He got the Democrats to stand, I’ve never been able to do that, I couldn’t believe it,” he added to rapturous applause.

Palace aides repeated that remark, clearly pleased with how well it went down with the President despite the subtle rebuttals of Trump’s administration.

It’s a sign of the warm rapport between the two men, not forgetting the Queen and First Lady, who get on famously. Throughout their joint engagements, the two women are often seen giggling and chatting away.

Discussing the strength in their personal relationship, the Palace aide said: "They get on very well. And it’s not just the King and the president. It’s all four of them with each other. The Queen and the president. The Queen and the First Lady. The King and the First Lady. The King and the president. It’s a pairing, sort of unlikely though it might be in many ways…

"I mean, given some of the issues that have presented themselves in the bilateral relationship, you’d think it might all be a bit tricky. But far from it. And the warmth that 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 alongside the more serious dialogue."

In the months leading up to the trip, the Palace and Downing Street held frequent discussions about its objectives, and the feeling is that these have all been met. He was able to emphasise the importance of economic, defence and cultural ties as well as reaffirm the people-to-people links between the two countries.

But the King had no time to dwell on the success of the four-day state visit, as after concluding engagements in Virginia on Thursday afternoon, he immediately flew to Bermuda for a royal visit.

Aides had hoped to shut their eyes and get some respite on the two-hour flight after a hectic few days that saw the King and Queen undertake 18 engagements between them.

But Charles, who is 77 and continued to undergo cancer treatment, had other ideas, and within 10 minutes he had already got changed into his arrival suit for Bermuda and was riffling through his file on Bermuda before turning to his state box.

One insider joked that there wasn’t even time to open a packet of nuts before the wheels touched down on the 21-square-mile archipelago.

But that perfectly sums up who the king is, driven by duty and refusing to let the inconvenience of his illness get in the way. A senior palace official said: “This is a man who is absolutely driven by duty. Driven by service."

And he was greatly invigorated by the overwhelming response to his whirlwind trip to Bermuda, which saw thousands of people come out to cheer and greet him across the island.

“The King has greatly enjoyed it,” the aide explained. “It has been very busy and the crowds have been so huge they have made us extremely late, but no one minds about that when you feel that you’re doing some good here, showing that this island matters to Britain, because it certainly matters to the King."

But as he arrived back in London last night , the King has his sights on the next goal. “He’s not a man to dwell long on what some may consider yesterday’s successes,” the senior aide said.

“He is always looking towards tomorrow’s opportunities, so generally we’ve tried to encourage him to feel positive - and he has, about the way that the American leg in particular has gone in particular, but his mind is already on what he can achieve next rather than what he achieved this week."

And speaking ahead of the King’s final three engagements in Bermuda, they added: “As long as it goes well today, we’ll sit on the plane and think that was a week well spent.”