Princess Margaret was one of the first royals to welcome Princess Diana when she became a member of the British royal family – but duty ultimately came calling.

On Tuesday, Diana’s biographer Andrew Morton told People magazine that at first, Queen Elizabeth II’s younger sister saw "a kindred spirit" in Diana, who struggled to fit in.

The 68-year-old recently wrote a new book titled, "Elizabeth & Margaret: The Intimate World of the Windsor Sisters," where he analyzed the lives and legacies of the royal siblings.

"Neither of them were hunting, shooting or fishing types," the author explained about Margaret and Diana’s instant connection. "Margaret put her arm around Diana. She could see they were two metropolitan princesses."

PRINCE HARRY, PRINCE WILLIAM WILL EVENTUALLY REPAIR THEIR RIFT FOR PRINCESS DIANA’S SAKE, ROYAL AUTHOR SAYS

Diana, Princess Of Wales, holding Prince Harry as she watched Trooping the Colour with Prince William and Princess Margaret from the balcony of Buckingham Palace.

Diana, Princess Of Wales, holding Prince Harry as she watched Trooping the Colour with Prince William and Princess Margaret from the balcony of Buckingham Palace. (Photo by Tim Graham Photo Library via Getty Images)

"Princess Margaret came to her rescue, suggesting to the Queen that Diana was having difficulties adjusting to her role and that she should cut her some slack," Morton wrote in his book, as quoted by the outlet.

"The Queen, who avoids family confrontation if at all possible, took her sister’s advice," he shared.

However, that all changed in 1995 when Diana gave a tell-all to Martin Bashir for Panorama. In the infamous interview, the Princess of Wales not only aired her grievances, but she also said "there were three of us in this marriage," referring to Prince Charles’ relationship with ex-girlfriend Camilla Parker Bowles.

According to Morton, that’s when "the guillotine really came down."

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According to biographer Andrew Morton, Princess Margaret (left) was very disappointed in Princess Diana's explosive interview with Martin Bashir.

According to biographer Andrew Morton, Princess Margaret (left) was very disappointed in Princess Diana's explosive interview with Martin Bashir. (Photo by Princess Diana Archive/Getty Images)

The author pointed out that Diana broke away from the royal family’s practice of keeping a "stiff upper lip." Instead, she detailed the breakdown of her marriage to Charles, her mental health struggles, the pressures of royal life, as well as the lack of support she felt within the family.

"[Margaret] had always defended the Queen and was always loyal to her," Morton told the outlet. "She felt Diana let her sister down. She was the Praetorian guard of her sister."

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Diana, who divorced Charles in 1996, died in a Paris car crash in 1997 as she was being pursued by paparazzi. She was 36. Margaret died in 2002 at age 71.

Charles, now 72, married Parker Bowles, 73, in 2005. She is now the Duchess of Cornwall.