Princess Diana’s brother has told the BBC exclusively that he is “telling the truth” about a phone call with King Charles in the days after his sister’s death.

Earl Spencer claims in a new book the then-Prince of Wales said “we’ll forget her soon enough” during an argument over whether Princes William and Harry should walk in Diana’s funeral procession.

In a strongly worded response, Buckingham Palace said the King was “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”.

But in his first interview since, Earl Spencer said: “I can tell you here and now, those are exactly the words he said.”

Reacting to the rare statement from the Palace, Earl Spencer said: “Well, is he furious? Is he furious or is he embarrassed?”

Speaking to the BBC at his Althorp estate, he added: “I’ve only ever had two calls on the line with Prince Charles at any stage of my life.

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

Pressed on whether he had evidence to back up the claim, he said he told family and friends at the time what had been said “because it was so monstrous”.

In response to the Palace disputing his account, Earl Spencer, who shares his first name with the King, said: “They’ve sort of smudged that one, haven’t they? They’ve said that recollections may vary.”

When asked which Charles the public was meant to believe, he said: “I’m telling the truth.”

Buckingham Palace declined to give a response to Earl Spencer’s fresh comments.

The remarks may bring a renewed focus to the war of words between the Palace and the King’s former brother-in-law.

In excerpts of his book published by the Daily Mail, Earl Spencer claims the phone call took place after he insisted Diana would not have wanted the young princes to walk in the procession.

In his BBC interview, he says the King “went ballistic” and “ranted for a while”, accusing his family of not having an adequate sense of duty.

“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 couldn’t say exactly what went through his mind at that moment as he was “just so stunned”, adding: “I just, after a stunned silence, I said, I cannot believe you just said that. And I hung up on him.”

Earl Spencer’s book, Swan Song: Diana, My Sister, is due to be published next week and is expected to be highly critical of the Royal Family.

In further extracts published on Thursday, he claimed how the King “sounded giddily elated” when he called him after Diana’s death in a car crash in Paris on 31 August 1997, alleging Charles sounded like a “lottery winner”.

Earl Spencer recounts being alerted to the crash by a phone call from his sister Jane in the middle of the night at his home in Cape Town, South Africa.

A call from Charles was among those that followed, and he alleges: “Unlike my other callers, who were stunned, barely able to articulate their condolences and offers of help, he sounded giddily elated – like a lottery winner, I remember thinking at the time.”

The death of Diana, a hugely popular figure, created a volatile atmosphere in the country at the time, with the royals accused of failing to show feeling and empathy, and seeming to be out of touch with the public mood.

Earl Spencer’s emotional and defiant speech at her funeral added to that cloud of grievance hanging over Diana’s death at the age of 36.

Prince Harry has previously said that walking behind his mother’s coffin aged 12 was something no child “should be asked to do”.

In 2017, he told the BBC he didn’t “have an opinion whether that was right or wrong”, but “looking back on it”, he was glad to have been part of the day.

The full interview with Laura Kuenssberg will be on BBC iPlayer on Sunday from 16:00 BST and on BBC One at 18:30 BST.