EXCLUSIVE: Prince William 'Sickened' by Dad King Charles' 'Evil Insult' About Late Mom Diana — After Charles Spencer's Bombshell Claims About Monarch

Prince William was reportedly “sickened” by bombshell claims from his late mother Princess Diana’s brother that his father, King Charles III, sounded “giddy” following her tragic death, RadarOnline.com can reveal.

Charles Spencer's new memoir, Swan Song: Diana, My Sister, leveled several incendiary claims about how then-Prince Charles reacted to Diana's 1997 death in a Paris car crash and a fight the two men had leading up to her royal ceremonial funeral.

Spencer wrote that after finding out over the phone at his home in South Africa that his sister was dead at the age of 36, "The phone rang constantly. Prince Charles caught me as I crossed the landing upstairs. Unlike my other callers, who were stunned, barely able to articulate their condolences and offers of help, he sounded giddily elated – like a lottery winner, I remember thinking at the time."

The explosive confession reportedly leveled Diana's eldest son.

"William is sickened by his father’s evil insult because, as we’ve seen in the past, William is fiercely protective of his mother — and the prince views her as untouchable," a palace insider claimed.

The source continued, "Diana means a lot to him, and he will protect her legacy at all costs."

"She was a much more present parent, while Charles was more concerned about the monarchy,” the insider claimed, then shared about William and his brother, Prince Harry. "Diana is on a pedestal; Charles is not, and it's easy to see how they will turn against him after those comments."

In an interview with CBS Sunday Morning ahead of the book's September 21 release, Spencer doubled down on the claim about Charles' reaction to Diana's death.

"Obviously I'm not justifying or criticizing. I'm just reporting that's the fact," he shared at the time.

Spencer believed Charles' allegedly bizarre reaction stemmed from Diana’s death clearing an easier path for the future king to eventually marry his longtime love, Camilla Parker Bowles.

Charles and now Queen Camilla eventually tied the knot in 2005.

Spencer made another sensational claim about Charles, saying the future monarch told him the royal family would "forget her soon enough" after the world's collective sadness from Diana's funeral died down.

In one passage, the 9th Earl Spencer wrote that the two men got into a furious row over whether or not then 15-year-old William and a nearly 13-year-old Harry should walk behind their mother's funeral procession on the way to the service at Westminster Abbey.

Spencer wrote that Charles briefly paused after he "went nuts" during the call.

"I assumed the pause signaled an effort to bring himself under control, but then he unleashed down the phone what I have always taken to be his pent-up contempt for Diana, and his horror that the world was so bowled over by her death," he continued.

Spencer alleged Charles "hissed" at him, then scoffed, "Rest assured, we'll forget her soon enough!"

After the explosive excerpt from Spencer's book was first serialized in a British tabloid, Buckingham Palace was quick to fire back with a statement suggesting Diana's brother's recollection of events may differ from what actually happened nearly 30 years ago.

The statement read, "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."