The senior royal official revealed the King more than understood the considerable risks involved but viewed the challenge as “a phenomenal opportunity” to strengthen transatlantic ties.

“What looked like risk and challenge was also a phenomenal opportunity. One that was grasped in both hands by the King,” a senior palace aide said.

The visit came at the Government’s request, with Downing Street hoping soft power might help repair relations strained by President Trump’s repeated media barrage of Sir Keir Starmer over the conflict in Iran, immigration, Net Zero and various other policies.

Liberal Democrat leader Sir Ed Davey had even called for the trip to be cancelled due to concerns that the King might be embarrassed.

But the relationship between the King and President Trump is notoriously warm, and this visit proved no different, with President Trump lavishing praise on the monarch repeatedly, even once insisting he was “jealous” of King Charles.

Palace sources disclosed that their private meeting in the Oval Office was littered with considerable laughter alongside intense and serious discussions.

The connection extended to Queen Camilla and First Lady Melania, creating what officials described as an unlikely but genuine friendship between the couples.

“They get on very well. And it’s not just the King and the President. It’s all four of them with each other,” a senior aide revealed.

“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.

“The warmth that you see in public is absolutely the warmth you see in private.”

Mr Trump repeatedly lauded the King as “fantastic” and “the greatest King” throughout the visit.

King Charles became the first British king invited to address a joint session of Congress, following only his late mother in receiving such an honour.

His speech touched on subjects where he and Mr Trump hold differing views, including the importance of Nato, support for Ukraine, and the value of European partnerships.

“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,” the aide explained.

The address resonated across party lines, with Republicans and Democrats rising to applaud on multiple occasions – something President Trump ironically admitted that even he has been unable to achieve.

The New York Times observed that beneath the King’s humour and diplomatic courtesy lay “some subtle rebuttals to Trump” regarding Britain and Nato, alongside references to checks and balances.

The Palace official said the monarch spoke as he did because “it’s a measure of how much he personally cares”.

The visit yielded concrete results for British interests, with Mr Trump announcing the removal of tariffs on Scottish whisky as a parting gesture.

The President said the UK Government had sought this change for some time, but “the King and Queen got me to do something that nobody else was able to do, without hardly even asking”.

“It’s not a competition between the King and the Government,” the senior aide insisted.

“The King is there to support the Government, to help the Government.”

The positive reception helped place the special relationship on firmer ground during the 250th anniversary year of American independence.

Royal officials believe the King and Queen helped advance Government objectives while cutting through “the noise of current affairs”.

Following the American leg, Charles travelled solo to Bermuda for a brief visit, celebrating the island’s culture and people, which concluded on Saturday.

The King continues to receive treatment for an undisclosed form of cancer, yet remains wholly committed to his duties.

“This is a man who is absolutely driven by duty. Driven by service,” the aide poignantly stated.

Enormous crowds greeted the monarch throughout his Bermuda engagements, causing significant delays to his schedule.

“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,” the aide added.

In a social media farewell, Charles thanked Bermudians for “such a wonderful few days and for the fond memories that will last a lifetime”.

Palace officials concluded the King is not one to dwell on achievements and is already focusing on his next diplomatic visit. Whether it will be as high-profile or successful is yet to be seen.