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.
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 Commonwealth Heads of Government Meeting (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", external, and that Britain "must face up to the past".
His pledge 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.
"Institutions are under strain across the globe," said a Palace spokesperson. "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".
Queen Camilla is with the King in Antigua, they have attended a church service on Sunday morning ahead of joining celebrations to mark Independence Day on the island.
