PRINCESS DIANA’S BROTHER felt it was his “duty… as an eyewitness to history” to write a new memoir chronicling the life of his sister, which has included a series of damning claims about Britain’s King Charles.

It comes after Charles Spencer accused the King of gaslighting him after Buckingham Palace pushed back against explosive claims that King Charles said “we’ll forget her soon enough” after Diana, Princess of Wales, died.

Diana’s brother Charles Spencer has made a series of claims about King Charles in a new memoir chronicling the life of his sister that prompted the response from Buckingham Palace, which said “the pain of fraternal grief can cloud reason”.

Charles Spencer told the BBC it was his ‘duty’ to write the memoir Jeff Overs / BBC Jeff Overs / BBC / BBC

On his motivations for writing the book, Spencer told the BBC it was his “duty… as an eyewitness to history”.

He said: “I’m aware of mortality and I wanted to just put things down once and for all that were true.

“This is not a threat, it’s a fact, I know lots of things about the King which I chose not to include in the book because they were not relevant.”

Spencer accused the King of gaslighting him and called the palace’s response a “rather poor imitation of the ‘recollections may vary’ quote” – late Queen Elizabeth’s response to the Duke and Duchess of Sussex’s allegations in their Oprah Winfrey interview, The Mail on Sunday reports.

He said: “It is not every day you wake up to find you’ve been gaslit by the King.

“It was astonishingly provocative and below the belt because it was essentially trying to say I’d lied without the King being able to deny what I’d said.

“I also thought it was a rather poor imitation of the ‘recollections may vary’ quote, which was a much more elegant way of putting things, but in these circumstances entirely out of place.”

In response to the claims made in the memoir, a Buckingham Palace spokesman said on Wednesday: “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 judgment and colour memory in ways others do not recognise, even many years after such a loss.”

Speaking about an alleged row over Diana’s funeral arrangements involving her sons, Spencer said: “(The King) went ballistic, would be the right term, and started really going for my family as a whole, about their sense of duty, not being adequate enough, and ranted for a while, I have no idea how long, but it was a significant period, and I just listened.

“And then he stopped, and I thought, ‘Oh now he’s going to pull himself together and say something more appropriate’.

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

King Charles is said to have made the comments about his ex-wife in a row with the earl over whether the then Prince William and Prince Harry should follow Diana’s funeral cortège.

On what William and Harry might think of the comments being brought up, Spencer added the words “will not be a surprise”, the BBC reports.

In another extract from the interview with the BBC, the earl said the relationship between the press and Prince Harry and Meghan “echoes” the way the media treated his sister.

File image of Megan Markle and Prince Harry Alamy Stock Photo Alamy Stock Photo

Spencer, who was in South Africa when his sister died, told the BBC he was initially told “it’s not too serious”, and after a number of calls, his sister Jane told him “I’m afraid she’s died”.

Meanwhile, Spencer kkktold the BBC he considers Swan Song: Diana, My Sister to be his final take on the situation.

He said: “I’m not here to bang on about Diana, this is my final word, celebration of her, and I will step off into the sunset now and not be available for comment.”