Princess Diana's biographer reveals the surprising question he never asked her

Diana, Princess of Wales (1961 - 1997) visits the Royal National Orthopaedic Hospital in Stanmore, Greater London, 1st July 1992
Princess Diana was interviewed by Andrew Morton for the 1992 biography Diana: Her True Story – In Her Own Words. (Getty Images)

Princess Diana's biographer Andrew Morton has revealed the surprising question he never asked her.

The late royal was famously interviewed for his 1992 book Diana: Her True Story – In Her Own Words, published the same year the mother-of-two officially separated from her husband Prince Charles.

Speaking to Hello! magazine's A Right Royal Podcast, Morton shared some of the process of writing the best-seller.

This included the one surprising thing the journalist, now 70, avoided saying to her in case it prompted her to withdraw.

Prince Charles and Diana, Princess of Wales (1961 - 1997) at Seville Expo '92, the Universal Exposition of Seville, Spain, 21st May 1992
The book was published the same year that the late royal officially separated from Prince Charles. (Getty Images)

He said: "The one question I never asked her was, 'Why are you doing it?' Because she might have thought to herself, 'Why am I doing it?'"

However, Morton insisted that the princess never "regretted" speaking to him for the book, and was always "relieved" to have revealed her experience.

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With this in mind, he suggested that Prince Harry, 38, whose memoir Spare went on sale earlier this week, would always be glad he shared his story.

The Duke of Sussex's book, which sold 1.4 million copies on its first day, has sparked a number of controversial headlines.

Andrew Morton attends the press night after party for
The journalist (pictured in 2018) said Diana never 'regretted' sharing her story with him. (Getty Images)

Morton revealed that Harry's mother, who passed away in 1997, became emotional as she read over the book before its publication.

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He said: "I remember occasions when Diana was in tears talking about some of the issues in her life, mostly her suicide attempts – and when I sent her to read through chapter six, she was in tears reading it because it just brought back so many memories.

"She did have a difficult childhood, an unhappy childhood, and talking about that as well it was like ploughing a field – it brought to the surface all kinds of unhappiness."

Watch: Prince Harry faces growing criticism over memoir revelations