The five words Prince Charles said that broke Princess Diana's heart.

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When Prince Charles and Princess Diana officially divorced in 1996, there was plenty of speculation about why.

But according to Diana’s biographer Andrew Morton, who wrote the book Diana: Her True Story, it was just five words from Prince Charles that spelled the beginning of the end for their relationship.

In his book, Diana said the first thing her husband said to her when Prince Harry was born was: “Oh God, it’s a boy.”

Diana had described her relationship with Charles as the closest they’d ever been in the later stages of her second pregnancy, but following this reaction their relationship unravelled.

She had known the gender of her baby, but kept it a secret from Charles because she knew he had always wanted a girl and would be disappointed.

Diana told Morton that Charles followed up his initial comment with something that again hurt her feelings: “And he’s even got red hair,” the Prince said – a trait that runs in Diana’s family.

Prince Harry on Princess Diana and Meghan Markle. Post continues below…

According to the book, Charles again voiced his displeasure at having another son at Prince Harry’s christening later that year.

“We were so disappointed — we thought it would be a girl,” he told Diana’s mother who apparently “snapped his head off” and told him how lucky he was to have a healthy child.

Diana described Harry to Morton as a “miracle” and a joy who helped her see through “total darkness”, but said his birth marked the beginning of her marriage to Charles going “down the drain”. She said the hurtful comments switched something off inside of her – there was no coming back from that.

In her 2010 book William and Harry, royal author Katie Nicholl described how Charles’ behaviour following Harry’s birth “crushed” Diana.

According to Nicholl, Charles snubbed his newborn son and Diana shortly after she had given birth.

“Diana was devastated when upon returning home to Kensington Palace, Charles sped off in his Aston Martin to play polo in Windsor Great Park.

Nicholl added that Diana later admitted: “Something inside of me died”.

Charles and Diana’s separation was announced in 1992, and their divorce was made official in 1996.