Earl Spencer says Princes William and Harry “won’t be surprised” at his explosive claims about King Charles’s reaction to the death of Princess Diana.

The earl, who was the younger brother of the late Princess of Wales, made the remarks during an interview with the BBC as controversy continues over extracts from his forthcoming memoir, Swan Song: Diana, My Sister.

According to Spencer, the then Prince Charles told him “we’ll forget her soon enough” following Diana’s death in 1997 after a disagreement over arrangements surrounding her funeral, The Mail on Sunday reports.

However, he said: “I don't like to talk about what I've been speaking about to William and Harry, but this will not be a surprise to them. I will tell you that these words will not be a surprise to them.”

The Earl has repeatedly stood by his recollection of the alleged exchange, despite a highly unusual intervention from Buckingham Palace.

In response to claims made in the memoir, a Palace spokesman said: “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.”

Spencer later accused the King of attempting to “gaslight” him through the statement, describing the Palace response as “astonishingly provocative” and saying it was effectively suggesting that he had lied about the conversation.

The disputed remarks allegedly followed an argument between Spencer and Charles over whether William and Harry, then aged 15 and 12 respectively, should walk behind their mother’s coffin during the funeral procession.

Spencer has claimed that Charles became angry during the discussion before making the comment about Diana being forgotten.

In the BBC interview, Spencer insisted he was “telling the truth” and said he had relayed the alleged comments to friends and family at the time because they were so “monstrous”.

He also argued that traces of the row could be seen in the eulogy he delivered at Diana’s funeral, in which he emphasised his sister’s sense of duty and pledged that her memory would endure.

The latest intervention is likely to intensify scrutiny surrounding Spencer’s memoir ahead of its publication.

The book contains a series of claims about the aftermath of Diana’s death and has already prompted an increasingly public dispute between her brother and Buckingham Palace.