Prince William has said that paparazzi used to spit at his mother Princess Diana to get her to react.

He said that people would be "utterly appalled" if they knew how his mother was treated 20 years ago by the press.

The Duke of Cambridge has spoken publicly for the first time about the impact his mother's death had on him and his brother Harry in an ITV documentary marking the 20th anniversary of her passing.

Prince William said that Diana's death was like "an earthquake" that shook their entire lives and took a long time to sink in.

"There's not many days that go by that I don't think of her, you know - sometimes sad, sometimes very positively," he said.

He described his mother as a "joker" who "really enjoyed laughter and fun" and said the only time "she ever cried about anything was to do with press intrusion."

Princess Diana was killed during a car crash in Paris in 1997, along with her friend Dodi Fayed and their driver Henri Paul. Their car was being chased by paparazzi on motorbikes when it crashed in a tunnel.

"If you are the Princess of Wales and you're a mother, I don't believe being chased by 30 guys on motorbikes who block your path, who spit at you to get a reaction from you... and make a woman cry in public to get a photograph, I don't believe that is appropriate," Prince William said in the documentary.

Prince William has worked hard to keep his own family out of the press, only allowing his children to be photographed at official royal outings.

Earlier this year he said he wanted his son Prince George "to grow up in a real, living environment."

"I don't want him growing up behind palace walls – he has to be out there," he said, admitting that "the media make it harder"

"But I will fight for them to have a normal life. One lesson I've learned is you never let them [the press] in too far because it's very difficult to get them back out again," he said.

On Saturday (22 July), the Duke and Duchess of Cambridge released their son's first official portrait to mark the occasion of his fourth birthday.

Princess Diana
Princess Diana holds Prince William, 6, and Prince Harry, 3, in Palma de Mallorca, Spain on 9 August 1987 Reuters