Prince William has been extremely candid about his late mother, Diana, Princess of Wales, ahead of the 20th anniversary of her death on 31 August, and has discussed her bulimia on camera for the first time.

The 35-year-old royal speaks of how proud he was of his mother for speaking about her mental health battle in a documentary made by former ITN newsreader Mark Austin – whose own 22-year-old daughter Maddy suffered with anorexia.

Called Wasting Away: The Truth About Anorexia – which airs on Channel 4 this Thursday at 10pm – William speaks with Austin and Maddy at Kensington Palace about Diana's illness.

The pair became acquainted when William wrote to him after reading a newspaper article by Austin about being unable to cope with his "dangerously ill" daughter and the family's struggle to get help for her.

The documentary shows Austin and his daughter watching Diana's famous 1995 interview with Martin Bashir in which she talks about Prince Charles' affair with Camilla and her struggles with an eating disorder.

She said: "I didn't like myself, I was ashamed I couldn't cope with the pressures. I had bulimia for a number of years, and that's like a secret disease. It's a repetitive pattern which is very destructive. It was my escape mechanism."

Princes Diana and Prince William
Princess Diana holds her two sons Prince William, 6, and Prince Harry, 3, (front) as they pose during the mornings's picture session on August 9, 1987 in Palma de Mallorca, Spain. Reuters

William, who is continuing his mother's legacy and challenging the stigma surrounding mental health, tells Austin: "We need to normalise the conversation about mental health. The fact that you are speaking out is incredibly brave."

Maddy tells the prince: "I was inspired by what your mum did," and William replies: "We need to be matter-of-fact about it, and not hide it in the dark where it festers."

When Austin tells William that he thought it was "amazing" how Diana spoke out on the issue 22 years ago, he asks: "Do you feel proud about that?"

Prince William
Prince William, the Duke of Cambridge Reuters

William says: "Absolutely. These are illnesses. Mental health needs to be taken as seriously as physical health."

Giving William some parental advice and admitting he took his eye off the ball with Maddy, Austin tells the royal: "I tell you what, William, keep an eye on Charlotte and George."

Bulimia was revealed as Diana's "secret disease" in 1992 in Andrew Morton's book Diana: Her True Story, with Morton explaining that she first struggled with the disease in 1981 and was still suffering it during the late Eighties when she sought treatment.

Because of Diana's courage to speak about and combat the eating disorder, this helped many others confront their issues, leading the movement to be known as the "Diana Effect".

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Mark Austin and daughter Maddy in The Truth About Anorexia Channel 4