'She was one of the naughtiest parents' Princes William and Harry open up about Princess Diana for ITV documentary

Princes William and Harry have opened up about Princess Diana as a mum for a new documentary coming to ITV this month, to mark the 20 year anniversary of her death in August.

In a new promo for the ITV documentary Diana, Our Mother: Her Life and Legacy, the princes open up about their mum, saying she was one of the 'naughtiest' parents.

In the 30 second clip, the brothers can be seen looking through a private family album assembled by their mother, reminiscing about some of their most joyful memories of their childhood.

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While looking at a photo of his mother holding him whilst pregnant with his brother, Prince William says: 'Believe it or not, you and I are both in this photograph, you're in the tummy!' The two royals have been getting increasingly more open about their struggle with grief following their mother's death, but Harry admits they've never spoken about Diana as a mum.

'This is the first time that the two of us have ever spoken about her as a mother', Harry says. Describing what Diana was like as a mum, William said: 'She was very informal and really enjoyed the laughter and the fun. But she understood that there was a real life outside of Palace walls.'

Harry, who was only 12 years old when Diana tragically died, adds that their mum was always smothering them with love: 'She was one of the naughtiest parents. She was our mum. She still is our mum. And of course, as a son I would say this, she was the best mum in the world.

'She smothered us with love, that's for sure.' Diana, Our Mother: Her Life and Legacy will air later this month to mark the 20th anniversary of Diana's death. The BBC will also be airing a documentary to mark the date, with the princes sharing more insight into their mother and childhood.

Speaking about the programme, ITV's head of factual Jo Clinton-Davis said: 'This new ITV film will offer viewers a fresh and revealing insight into Princess Diana through the personal and intimate reflections of her two sons and of her friends and family, many of whom have never spoken before, to bring together a definitive portrait of a unique person who touched the lives of millions.' The film's producer, Nick Kent, said: 'This film will show Princess Diana in a way she has never been seen before, through the eyes of the two people who knew her best.'

Will you be watching Diana, Our Mother: Her Life and Legacy? Let us know in the comments!