King Charles III and Prince William are reportedly furious after Princess Diana's brother, Charles Spencer, published a memoir containing damaging allegations about the monarch. The book, titled "Swan Song: Diana, My Sister," was released on Tuesday and includes Spencer's account of a heated argument with then-Prince Charles regarding the 1997 funeral procession for Diana. Spencer claims that Charles hissed, "Rest assured, we'll forget her soon enough," during the dispute over whether William and Prince Harry should walk behind their mother's coffin. He also alleged that Charles sounded "giddily elated" after Diana's death in a car crash at age 36. Royal expert Kinsey Schofield told Fox News Digital that the King's anger stems from what she describes as an attack on his character nearly 30 years later, while Prince William feels a sense of betrayal from an uncle who had previously promised to protect them. Buckingham Palace issued a rare statement noting that the pain of fraternal grief can cloud reason and affect memory. A spokesperson for the palace declined to comment further, while Kensington Palace stated it does not comment on books. Spencer has defended his account in an interview with the BBC, asking if the King is furious or embarrassed. Author Christopher Andersen, who wrote "The Day Diana Died," told Fox News Digital that he was surprised by the allegations. Andersen said that hundreds of sources he interviewed in London and Paris remembered Charles as devastated and crushed by the news. He recalled that Charles let out a howl upon hearing the news at Balmoral and wept openly when he saw Diana's body in Paris. Andersen suggested that Spencer may have misinterpreted the words of a man on the verge of a nervous breakdown during a period of immense stress. Andersen noted that Queen Elizabeth II had initially refused to fly the flag at half-staff or return from her summer vacation, leaving Charles to force the issue regarding public respect for Diana. Despite the controversy, Spencer said he wanted to record his own thoughts to avoid reading others' opinions based on untruths. The book is being translated into 12 languages. British royal expert Hilary Fordwich said William is "absolutely furious" but has maintained public silence. Broadcaster Helena Chard stated that the palace is "rattled, but they're not panicking" and is presenting a united front by continuing its duties. A source told Radar Online that Queen Camilla is trying to lower the temperature behind palace doors, believing that reopening old wounds gains nothing.