In the latest documentary commemorating the 20th anniversary of Princess Diana’s death, an angry Prince William slams photographers who stalked his mother as “a pack of dogs”.

He also talks for the first time about the “damage” caused to him and his brother, Prince Harry, by seeing their mother continually distraught about her “appalling” treatment.

William 35, recalls in new BBC documentary Diana, 7 Days, that “every single time” the princess left her Kensington Palace home, waiting paparazzi “harassed her, called her names, spat at her, trying to get a reaction.

“To get that photograph of her lashing out. Get her upset.”

The result? “Very sadly, a lot of my memories revolve around trying to cheer her up.”

For Harry, 32, “knowing there was nothing we could do” in defence of their mother “was very hard”.

In the documentary, he describes one “particularly bad” day when Diana was taking her boys to a tennis lesson.

Cars and motorbikes were following the family, and a “fed up” Diana stopped the car, and “went running up to these guys and shouted and screamed at them while they took photographs of her,” Harry says.

“That lasted for about five minutes … all I could hear was screaming.”

When she got back in the car, “she couldn’t even talk to us,” says Harry says. “She was constantly crying.

“William and I looked at each other and stared out of the window. Is this the way it was supposed to be for the rest of our lives?”

Diana “cried more to do with the press intrusion than anything else in her life”, William says in the documentary, which premieres in the UK on August 27.

“We’d go looking for her, to talk to her, to play and she’d be crying — and when that was the case, it was to do with the press.”

“The damage for me, being a little boy being eight, nine, 10 and wanting to protect your mother and finding it very difficult seeing her upset,” he adds.

Diana died on August 31, 1997, aged 36, while being followed by photographers through the Pont de l’Alma tunnel in Paris.

“One of the hardest things to come to terms with,” Harry says, “is the fact that the people who chased her into the tunnel were the same people that were taking photographs of her while she was still dying on the back seat of the car.

“William and I know that – we have been told that numerous times by people who know that was the case.

“She had quite a severe head injury but was still very much alive on the back seat and those who caused the accident instead of helping were taking photographs of her dying.”

Former army captain Harry says Prince Charles “was going through the same grieving process” as his sons, but he “was there for us — he was the one out of two left, and he tried to do his best and to make sure that we were protected and looked after”.

The young prince’s initial reaction to losing his mother was “disbelief”, recalls Harry.

“I remember feeling completely numb, disorientated, dizzy. And you keep asking yourself, ‘Why me?’”