EXCLUSIVE: The Moment Princess Diana Decided Her Marriage to Charles Was Over Revealed

Princess Diana told her brother over lunch she was going to divorce then-Prince Charles – marking the moment she believed her troubled royal marriage was finally beyond saving, RadarOnline.com can reveal.

Charles Spencer, 62, recalls the extraordinary conversation in his memoir Swan Song, revealing Diana made the declaration while they were eating at her favorite London restaurant.

Diana, who died aged 36 in 1997, made a startling claim about her husband's relationship with an unnamed member of staff, although Spencer says he never investigated what she meant or why she believed it.

A source familiar with Diana's stormy marriage to Charles claimed to Radar: "For Diana, this was the moment when the marriage was over in her own mind. Whatever hopes she had once possessed of somehow repairing things with Charles had effectively disappeared.

"What stands out is how matter-of-factly she apparently told her brother she was getting divorced. By then she had endured years of unhappiness, public scrutiny and the knowledge that Charles' relationship with Camilla remained part of their lives."

"The marriage had begun as this extraordinary global fairy tale, but Diana had reached the point where she no longer believed that fairy tale could possibly be salvaged," the insider noted.

Spencer writes Diana told him over their lunch: "I'm going to get divorced."

Diana married the future King Charles III in July 1981 at St Paul's Cathedral before a global television audience. She was 20, while Charles was 32.

Their sons, Prince William, now 44, and Prince Harry, now 42, were born in 1982 and 1984 respectively, but behind the public image of royal domesticity, the marriage became increasingly unhappy.

Charles maintained a close relationship with Camilla Parker Bowles, now Queen Camilla, 79, whom he had known before marrying Diana.

Both Charles and Diana subsequently acknowledged adultery during their marriage. Their difficulties became increasingly public during the early 1990s.

The pair formally separated in December 1992, with the then-prime minister of the UK, John Major, announcing the decision in the House of Commons.

Diana subsequently spoke openly about the collapse of the relationship during her explosive 1995 interview with BBC Panorama.

Referring to Charles' relationship with Camilla, she famously said: "Well, there were three of us in this marriage, so it was a bit crowded."

Charles and Diana's divorce was finalized in August 1996, leaving her free of the institution which had defined much of her adult life.

Almost exactly a year later, on August 31, 1997, Diana was fatally injured when the Mercedes carrying her and Dodi Fayed, 42, crashed in the Pont de l'Alma tunnel in Paris.

Swan Song revisits Spencer's memories of his sister as the 30th anniversary of her death approaches.

Sources have told us Harry approved of Spencer's book and wants to interview him for a documentary on his mother's life and death he is said to be planning to release in 2027 to mark three decades since her passing.

Insiders have added that his estranged brother William is "fuming" at the plan.

One claimed: "He views it as totally bad taste and wishes Harry would let Diana rest in peace, and not use her for a streaming project.

"If Harry releases this, it will make the brothers' rift even deeper, and William could see it as a turning point in their broken relationship from which there is no going back."