Diana's brother insists: Charles told him she'd soon be forgotten. The palace doesn't deny it.

Earl Charles Spencer, brother of the late Princess Diana, stuck in a BBC interview published on Sunday to the claim that prompted Buckingham Palace to respond last week: that in September 1997 Prince Charles, now King Charles III, told him by phone that Diana would soon be forgotten.

Spencer described the sequence of events to presenter Laura Kuenssberg. He had been talking to a palace representative about the funeral procession and said Diana would not have wanted William and Harry to walk behind her coffin. Soon afterwards the prince allegedly called him. In Spencer's account, the prince cited his own case: he had walked behind Lord Mountbatten's coffin at the age of thirty. Spencer replied that Mountbatten was not his mother. The prince then allegedly accused him of lacking a sense of duty and said the sentence: "We'll forget her soon enough." Spencer says that after a stunned silence he said he could not believe what he had just heard, and put the phone down.

The palace answered the claim from the book with a statement that nobody in London appears to have read as a denial.

"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 color memory in ways others do not recognize, even many years after such a loss."

In plain translation: we do not comment on books as a matter of principle, but the king is aware that the pain of losing a sister can cloud reason, affect judgment and color memory in a way others do not recognize. So the statement does not say the sentence was never spoken. It says Spencer may not remember correctly.

Spencer told the BBC he remembers it precisely, and offered as his argument that he had spoken to Prince Charles on the phone only twice in his life.

"I was hyper-alert to what he was saying, and those are precisely the words he said. And I promise that to you now. Genuinely, that is what he said."

Asked whom the public should believe, him or the king, he answered that he is telling the truth, and asked what else would have made him put the phone down that day. He added that at the time he told close friends and family about the "monstrous" remark. He told Britain's Mail on Sunday that the king's statement branded him a liar, without the king himself denying anything: "It is not every day you wake up to find you've been gaslit by the king."

What can be checked here and what cannot

Three things can be checked. The book Swan Song: Diana, My Sister comes out on Tuesday, 22 September. The British edition is published by Michael Joseph and the American one by Penguin Press, and it will be translated into twelve languages. In August the publisher released Spencer's explanation of why he is writing it: "The aim of this book is to tell the truth about Diana from her brother's perspective, just as I attempted to do when speaking at her funeral." And the palace stayed silent after its first statement; on Sunday it again declined to comment.

The core cannot be checked. It is the content of a private phone call from twenty-nine years ago. There is no recording and no third person on the line. There are two memories, and one of them is presented publicly through a media statement, not in the first person. Spencer admits this limit himself at one point: when Kuenssberg asked whether he could be sure what the prince was thinking at the time, he replied that he cannot be sure and that it is his supposition based on the tone.

The same goes for another claim from the book that the presenter asked Spencer about: that Diana allegedly told him her husband was in love with his valet. Spencer himself calls it unlikely, but included it in the book anyway. Asked whether he did so to help the book sell, he answered that it was part of the conversation in which his sister told him her marriage was over, and that she deserves honesty.

The rest of the interview is less contentious and more useful. For the first time, Spencer described how he learned of his sister's death: in South Africa, his sister Jane Fellowes called him in the middle of the night and told him the accident was "not too bad". More and more serious calls followed, the last with the news that Diana had died. He expressed concern that Prince Harry has no police protection, since his sister died in circumstances that could have been prevented. He also said he takes neither side in the dispute between William and Harry.

No new facts are likely before Tuesday. There will be a book and two memories of one phone call.