Prince Harry reveals regrets over final phone call with mother Diana and his struggles dealing with her death in ITV documentary

“The first time I cried was at the funeral on the island (at Diana’s family home at Althorp). And, probably, since then maybe once. So you know there's a lot of grief that still needs to be let out.

“I was so young. I grew up sort of thinking that not having a mum was normal. I think it was a classic case of don’t let yourself think about your mum and the grief and the hurt that comes with it, because it’s never going to bring her back and it’s only going to make you more sad.

“People deal with grief in different ways… and my way of dealing with it was by just basically shutting it out, locking it out.”

In Monday night’s documentary, Diana, Our Mother: Her Life and Legacy Harry and his brother William recall the final phone conversation they had with their mother hours before she died.

They were on holiday in Balmoral and busy playing with their cousins. They both struggle with the knowledge that the conversation was a hurried one.

Says Harry: “I can't really necessarily remember what I said, but all I do remember is… regretting for the rest of my life how short the phone call was. And if I'd known that that was the last time I was going to speak to my mother the things that I would have said to her. Looking back on it now, it's incredibly hard. I have to sort of deal with that for the rest of my life. Not knowing that that was the last time I was going to speak to my mum, and how differently that conversation would have panned out if I'd had even the slightest inkling that, you know, that her life was going to be taken that night.”

Prince William reveals in the film how he keeps the memory of his mother alive for his son and daughter, George and Charlotte. “I do regularly, putting George or Charlotte to bed, talk about her and just try and remind them that there are two grandmothers, there were two grandmothers in their lives, and so it's important that they know who she was and that she existed.”

That said, he jokes that she would have been a “nightmare” grandmother. “She'd love the children to bits, but she'd be an absolute nightmare. She'd come in probably at bath time, cause an amazing amount of scene, bubbles everywhere, bathwater all over the place and then leave.”

William goes on to say: “There’s not many days that go by that I don’t think of her. Her 20th anniversary year feels like a good time to remember, you know all the good things about her and hopefully provide maybe a different side to her that others haven't seen before.”

That “different side” includes a very un-royal mischievous streak that saw William surprised by three of the most beautiful women in the world.

He recalls returning from school to be confronted by Cindy Crawford, Christy Turlington and Naomi Campbell who had been invited by Princess Diana to welcome him home.

“I was probably [a] 12 or 13 year old boy who had posters of them on his wall. And I went bright red and didn't quite know what to say and sort of fumbled, and I think I pretty much fell down the stairs on the way up. I was completely and utterly sort of awestruck. But that was a very funny memory that - that's lived with me forever about her loving… and embarrassing and sort of, you know being the sort of the joker.”

Her sense of fun – and sauciness - extended to the choice of cards she would send William. “She was a massive card writer. She loved the rudest cards you could imagine. And I would be at school and I'd get a card from my mother. Usually she found something, you know, very embarrassing, you know, a very funny card, and then sort of wrote very nice stuff inside. But I dared not open it in case the teachers or anyone else in the class had seen it.”

Prince Harry, who was 12 when his mother died, says he remembers the love that she exuded. “It was that love that, even if she was on the other side of a room, you, as a son, could feel it.”

He also speaks of her willingness to bend and even break the rules.

“One of her mottos to me was, you know, ‘you can be as naughty as you want, just don't get caught’. She was one of the naughtiest parents. She would come and watch us play football and, you know, smuggle sweets into our socks. Our mother was a total kid through and through.”

“All I can hear is her laugh in my head. And that sort of crazy laugh of where there was just pure happiness shown on her face.”

The two princes have also given interviews for another TV documentary to be screened close to the anniversary of Princess Diana’s death on 31s August. The BBC’s film will feature them talking about the immediate aftermath of losing their mother.

*Diana, Our Mother: Her Life and Legacy is on Monday at 9pm on ITV