Prince William feels proud of his mother for having the courage to discuss her battle against bulimia.

Prince William

Prince William

The 35-year-old royal features in a documentary by former ITN newsreader Mark Austin, whose daughter also struggled with the eating disorder, and William wrote to him after reading about her struggles, admitting he was moved by Austin's story.

In the Channel 4 documentary 'Wasting Away: The Truth About Anorexia', Prince William is seen talking to Austin and his daughter Maddy about Princess Diana's troubles.

And Maddy admits to being inspired by the late Princess, who discussed the problem in her 1995 BBC interview with Martin Bashir.

In the much-discussed interview, the British royal 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."

And in the new documentary, Prince William speaks to Maddy about her past troubles, saying: "We need to normalise the conversation about mental health. The fact that you are speaking out is incredibly brave."

To which she replies: "I was inspired by what your mum did."

Then, William says: "We need to be matter-of-fact about it, and not hide it in the dark where it festers."

Asked whether he feels proud of his mother for acknowledging her eating disorder, Prince William added: "Absolutely. These are illnesses. Mental health needs to be taken as seriously as physical health."