Princess Diana’s brother has insisted he was “telling the truth” after making explosive claims about King Charles.

In Earl Spencer’s soon-to-be-released book, he claims King Charles said “we’ll forget her soon enough” while disagreeing about whether or not Prince Harry and Prince William should participate in Princess Diana’s funeral procession.

Earlier this week, Buckingham Palace released a rare and scathing statement about Earl Spencer’s claims.

“While we do not comment on books as a matter of principle, His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgement and colour memory in ways others do not recognise, even many years after such a loss,” it read.

While promoting his book, Swan Song: Diana, My Sister, Earl Spencer questioned the statement in a sit-down interview.

“Well, is he furious? Is he furious or is he embarrassed?” he said to the BBC.

“I can tell you here and now, those are exactly the words he said.”

He later added, “They’ve sort of smudged that one, haven’t they? They’ve said that recollections may vary.”

Princess Diana was 36 when she was killed in a car crash in Paris in August 1997. Her partner Dodi al-Fayed also died after their vehicle crashed while being pursued by paparazzi photographers on motorbikes.

The 62-year-old said he had only ever had two phone conversations with King Charles.

“I was hyper alert to what he was saying and those are precisely the words that he said and I promise that to you now,” he said.

“Genuinely, that is what he said. I don’t hear him saying he didn’t say that.”

When he was pressed for evidence of the explosive claims, Earl Spencer said he had told friends and family about the King’s comment “because it was so monstrous”.

“I’m telling the truth,” he added.

Princess Diana’s brother provided more information and context about his alleged interaction with King Charles, adding that the royal “went ballistic” and “ranted for a while”.

“And then he stopped and I thought, oh now, now he’s gonna pull himself together and say something more appropriate,” Earl Spencer said.

“And then, he said, ‘we’ll forget her soon enough’.”

He said he was “just so stunned” and told the King he could not believe the comment and hung up on him.

Swan Song: Diana, My Sister is expected to be published next week and will provide a personal account of his sister’s life and the events surrounding her death.