The 'joke' straight after Prince Harry's birth that always stuck with Princess Diana.

Princess Diana continues to capture the fascination of the public, almost 20 years after her tragic death in 1997.

In the lead up to the 20 year anniversary of her death, new information around the state of her relationship with Prince Charles has surfaced.

As part of Andrew Morton’s reissued unauthorized biography, Diana: Her True Story, Morton has released never-before published material about the downfall of Charles and Diana.

In the book, he writes that after suffering a miscarriage a year earlier, Diana was thrilled to be pregnant once again in 1984, however, Charles “desperately” wanted their second born to be a girl. He was reportedly very “dismayed” to be having another boy.

After nine hours of drug-free labour, Prince Henry Charles Albert David Mountbatten-Windsor (aka Prince Harry) was born. But immediately after the birth, Prince Charles made two comments that always stuck with Diana.

“His first comment was, ‘Oh God, it’s a boy.’ His second: ‘And he’s even got red hair,'” Diana told Morton.

Harry’s hair colour has led to much speculation over the years that he is not in fact Charles’s son, despite the trait appearing frequently on the Spencer side of the family.

It was reported that Charles voiced his displeasure at having another son at his christening later that year.

“We were so disappointed — we thought it would be a girl,” he told Diana’s mother.

Charles was reportedly very disappointed to be having another boy. Image via Getty.

According to the tell-all book, the fractures around Charles' and Diana's marriage began to appear not long after the birth of their second child.

Diana admitted that she and Charles were "the closest we've ever, ever been and ever will be," in the lead up to Harry's birth.

"Then, suddenly, as Harry was born, it just went bang, our marriage. The whole thing went down the drain."

This was probably due to Charles resuming his affair in Buckingham with Camilla Parker-Bowles.

"By then, I knew Charles had gone back to his lady," Diana said of their affair.

Prince Charles and Princess Diana would divorce more than a decade later in 1996.