Buckingham Palace has issued a rare statement responding to a claim made in an upcoming book by Charles Spencer, the brother of the late Princess Diana. The allegation, published in a preview by the Daily Mail, concerns remarks allegedly made by King Charles III, then Prince of Wales, in the days following Diana's death in 1997.

According to an excerpt from Spencer's book, "Swan Song: Diana, My Sister," Charles III told him during a disagreement over funeral arrangements, "rest assured, we'll forget her soon enough." This claim has prompted an unusual public comment from the typically reserved palace.

A spokesperson for Buckingham Palace stated to ABC News, "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." This response indicates the palace is aware of the book's contents and the specific allegation.

The palace's decision to comment is a departure from its usual practice of not addressing books or reports concerning royal family members. For instance, Buckingham Palace remained publicly silent following the release of Prince Harry's 2023 memoir, "Spare," which contained numerous claims about King Charles III and other royals.

Princess Diana died on August 31, 1997, at the age of 36, after a car crash in Paris. Her younger brother, Charles Spencer, is an author and historian who delivered the eulogy at her funeral. His upcoming book is expected to recount Diana's life from his perspective.

The palace's statement, while not directly confirming or denying the alleged quote, acknowledges the potential for grief to impact memory and perception. This nuanced response suggests an effort to address the claim without engaging in a direct public dispute over the historical event.

ABC News has reached out to Spencer's publisher, Penguin Random House, for further comment. The anticipation surrounding Spencer's book highlights the enduring public interest in Princess Diana and the complex relationships within the royal family.

This rare public comment from Buckingham Palace underscores the sensitivity of the allegations and the potential impact of Spencer's book on public perception of the royal family's past actions and sentiments.