The British government is increasingly using King Charles as "a secret weapon" to handle sensitive diplomatic situations, according to a senior royal aide following a challenging trip by the monarch to the Caribbean.

The visit, which included a meeting of the Commonwealth of Nations headed by Charles, was dominated by discussions over slavery, specifically whether Britain and the king ought to apologise for the transatlantic slave trade and offer reparations.

In line with British government policy, the king made no apology and offered no financial compensation in his speeches, though his personal sorrow regarding the horrors of slavery was widely welcomed by leaders of the 56-nation Commonwealth as a positive step.

In unusually candid comments, the senior aide described the tour as complex and challenging, but noted the king "had demonstrated his statecraft both by word and deed".

"It’s a great secret weapon to send the king," the aide told reporters anonymously, adding that the monarch was "able to reach the parts that, dare I say, politicians probably couldn't quite have reached".

While serving as Britain's symbolic head of state, the monarch is constitutionally required to stay out of political affairs.

However, as British global influence has diminished in recent times, ministers have frequently turned to the sovereign to lead diplomatic strategies.

Amid tensions with US President Donald Trump, most recently over the Iran war, the government called upon Charles — who became king in September 2022 following the death of his mother Queen Elizabeth — to help smooth ties. The king hosted Trump on a state visit and later paid a reciprocal state visit to the US.

"We’re just in the fifth year of the reign and I think what we're seeing is a very clear pattern emerging of the government asking the king to head into quite politically charged situations, quite challenging situations," the senior aide said.

The aide noted Charles relishes difficult assignments, adding that "refreshing and reinvigorating relationships with nations where there have been some complex political issues at play, that is what he lives and breathes for".

The tour stands as a highlight during a tough year for the royal family, following a biography of Charles' first wife, Princess Diana, written by her brother, which contained criticism of Charles' response to her death.

Meanwhile, Charles' younger brother Andrew Mountbatten-Windsor continues to generate negative headlines as he remains under police investigation - Andrew denies any wrongdoing - while the return to Britain of the king's younger son Prince Harry has brought further complications.

Although Charles remains head of the Commonwealth and head of state for 15 nations, including Canada and Australia, the aide acknowledged that situation could change, noting Barbados became the latest nation to ditch the monarchy in 2021.

The official explained that future arrangements remain entirely a matter for individual nations and the Commonwealth to decide.

"The (royal) family serves when it’s wanted and if not, then a new arrangement can be found," the aide added.

Charles, who turns 78 this month, is still recovering from cancer, but the aide said concerns about his age and health were best answered by the sight of Charles waving cowbells as he danced down a street in the Bahamas in 40 degrees Celsius (104 degrees Fahrenheit) heat and his good humour after being caught in a tropical rainstorm.

"This is literally what he was born to do. He's very good at it, and he enjoys the reaction he gets from people," the aide said.