Britain's King Charles III (Photo by Cristian Cristel/Xinhua)

King Charles III is to speak with "unflinching honesty" and "sincere sorrow" on the lasting pain of the transatlantic slave trade, during the final leg of his Caribbean tour in Antigua, Buckingham Palace has said.

The King will address Commonwealth leaders on Tuesday in a speech which will touch on the issue.

Prime Minister Andy Burnham said earlier he was "open" to the possibility of the UK formally apologising for its role in slavery - a change in the government's stance.

It is understood the Palace was told in advance of Burnham's comments in an opinion piece on Friday night, and the King has not made any changes to his speech in light of them.

The British government has never formally apologised for slavery or offered to pay reparations.

Burnham has previously stuck with the policy of his predecessors, and stopped short of offering any consideration of reparation payments.

In an address to the nation to mark Antigua and Barbuda's independence day, Prime Minister Gaston Browne said the history of slavery was painful and "its consequences have not disappeared with the passage of time".

He said the country's pursuit of reparatory justice deserved "seriousness, sensitivity and sustained international engagement".

Browne went on to say his government was having these conversations in bilateral discussions with the UK and other Commonwealth countries at the Commonwealth Heads of Government Meeting (CHOGM).

Conservative Party leader Kemi Badenoch described "endless talk about apologies" as a "distraction" from solving current problems.

She told BBC Sunday with Laura Kuenssberg: "The transatlantic slave trade happened centuries ago. It is time for the Commonwealth to start looking forwards, not endlessly backwards."

The King will make his speech at the CHOGM.

He "will not shy away from confronting the most shameful aspects of the past during this visit", a Palace spokesperson said.

"You may therefore expect him to address them in his speech with unflinching honesty, sincere sorrow, and respect for the wider discussions that will be taking place between relevant governments on this issue at CHOGM and beyond."

When asked about the King's reaction to the prime minister's comments, the spokesperson said it would "not be appropriate" for the King to address matters of policy relating to "the UK government's position".

The Caribbean tour has again thrust King Charles into the difficult debates over the UK's colonial legacy in the region.

On his first stop in the Bahamas, the King delivered a speech where he spoke of the "suffering of the past".

In Guyana, he visited the site of a Slave Rebellion where hundreds of enslaved people were killed.

Buckingham Palace said it was a moment for the King to learn more about the history of slavery and resistance. His engagements have also covered the environment, opportunities for young people and trade relationships with the UK.

Royal sources have stressed that the decisions around making an apology are a matter for government, not the Palace.

"His Majesty is well practised on the diplomatic tightrope," a Royal aide said. "He is up for challenging visits with complex issues and relationships in play. He sees it as his role to 'bat for Britain', the Realms and wider Commonwealth."

In a piece for the Guardian published on Friday night, Burnham described the transatlantic slave trade as a "brutal and barbaric violation" , and that Britain "must face up to the past".

His separate pledge in his article to strengthen the work of the Commonwealth as an organisation will be welcomed by King Charles, who views the collection of 56 countries as a powerful vehicle for change.

The King will be giving the opening speech at CHOGM, which gathers leaders from those 56 states every two years in a different part of the Commonwealth.

Each biennial meeting has a theme and this year's is officially called Accelerating Partnerships and Investment for a Prosperous Commonwealth.

Queen Camilla is with the King in Antigua and, as the couple attended a church service on Sunday morning ahead of joining celebrations to mark Independence Day on the island, there was a reminder of what the intended focus of the week will be.

Bishop of North Eastern Caribbean and Aruba, Ernest Flemming, told the congregation the "prosperity" of the Commonwealth must be a "shared prosperity".

"True partnership requires mutual respect, it requires listening, it requires fairness."

A Buckingham Palace spokesperson said: "Institutions are under strain across the globe. The Commonwealth has scale and breadth, but is also small enough for leaders to know each other and co-operate together."

The King, they said, "contributes ideas and encouragement on a wide range of issues, delivering practical results which help improve the lives of millions".

Later ion Sunday, the King was left drenched and without an umbrella when a torrential downpour interrupted an Independence Day ceremony.

Queen Camilla was swiftly given an umbrella when the rain started, while the King was seen gesturing to be given one of his own.

It has been a difficult few months for the King, with developments surrounding his brother, Andrew Mountbatten-Windsor , and the publication of a book by his former brother-in-law, Earl Spencer.

In addition, his son the Duke of Sussex made a surprise move back to the UK with his wife Meghan and their two children in August.

Asked if the King was confident that his approach to family matters had been the right one in terms of public attitudes towards the institution, the response was concise. "Yes," a senior aide told the BBC. "That is why he takes the actions he does - and has done."

On the serious allegations that have emerged about Meghan's father , Thomas Markle, the Palace did not want to get involved publicly: "We wouldn't comment on private family matters."

The King is also still being treated for cancer. He was diagnosed in early 2024 and the Palace says he remains on a "positive trajectory".

In hot and humid conditions, he will soon celebrate his 78th birthday and will have carried out about 36 engagements by the end of this nine-day tour of the Caribbean.