Prince William reveals the depth of his antipathy towards the paparazzi in the programme, recalling that photographers once spat at his mother to try to elicit a reaction.
He said people would be "utterly appalled" if they knew "exactly what went on" 20 years ago.
Described by her brother the Earl Spencer as the "most hunted person of the modern age", Diana was killed in a car crash, along with her friend Dodi Al Fayed and their driver Henri Paul, after being pursued through Paris by a pack of paparazzi in 1997, reports Daily Mail.
William says: "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.
"I sadly remember most of the time she ever cried about anything was to do with press intrusion. Harry and I, we had to live through that."
William's hostility towards the media was apparent from an early age. During a skiing holiday in Austria when he was 11, he turned on a group of photographers and had to be restrained by detectives.
In recent years, particularly since marrying and having children, he has been keen to shield his family from media exposure.
In September 2012, William and his wife said they were "hugely saddened" by a "grotesque" invasion of their privacy, after topless photographs of the Duchess were published by a French celebrity magazine. The couple are still seeking damages.
Royal aides called the photographs "reminiscent of the worst excesses of the press during the life of Diana".
William has called the photographs "particularly shocking" and said that they caused him particular distress because of the way his mother died.
Two years ago Kensington Palace issued an unprecedented warning to paparazzi, accusing photographers of harassing Prince George, and using "increasingly dangerous tactics" to obtain covert images of the third-in-line to the throne.
Princess Diana resting her head in her hands whilst sitting on the steps of her home at Highgrove, Gloucestershire. Photo / Getty Images
Paparazzi intent on selling expensive images to foreign publications have hidden in car boots and among sand dunes, and have used other young children as bait to draw their "No 1 target" into view, the palace said in an unusually strongly worded open letter.
But William and Kate praised the UK press for refusing to publish pictures they had taken.
Earlier this year, William said: "I want 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.
"The media make it harder but I will fight for them to have a normal life.
"One lesson I've learnt is you never let them [the press] in too far because it's very difficult to get them back out again.
"You've got to maintain a barrier and a boundary because if you cross it, if both sides cross it, a lot of pain and problems can come from it."
Recalling the infamous skiing trip to Lech in Austria, photographer Jayne Fincher tells the programme that some paparazzi reneged on a deal to simply picture the Princes and their mother at a photocall on the first day of their holiday.
Instead, she says, the pack pursued them "through the town to a sweetshop". Fincher adds: "The photographers were running everywhere. It was like rats running everywhere.
"They all wanted to be in their face, right up to them, pushing and shoving, they were fighting with each other. Photographers were falling over. And the boys were very frightened. I was in the shop when she [Diana] came in... and she was sort of exasperated by it [as if she was thinking] what, what are they doing? I've kept my part of the deal, we did our photocall, why are they all doing this?"
Pursued by the snap pack to the very end
The night of August 31, 1997 shows Diana Princess of Wales (head turned away in backseat), her bodyguard Trevor Rees-Jones (left) and driver Henri Paul. Photo / Getty Images
Diana was pursued by paparazzi from the moment her engagement to Prince Charles was revealed until her tragic death.
A London inquest in 2008 found the antics of a "pack" of French freelance photographers who chased her Mercedes on motorbikes was a contributory factor in the fatal accident - but the main cause was 'grossly negligent driving' by chauffeur Henri Paul.
A key factor was that neither Diana, her friend Dodi Al Fayed nor Paul were wearing seatbelts. All three were killed, while seatbelted bodyguard Trevor Rees-Jones survived.
Witnesses who arrived on the scene moments after the head-on collision with a concrete pillar recalled having to push through paparazzi to help the injured in the car.
Kate was a target too
Kate is 'papped' outside her home in 2007. Photo / Getty Images
The intrusive behaviour of the paparazzi around Kate Middleton when she was first linked with Prince William drew inevitable comparisons with Diana.
In October 2005, after the publication of a photo of her on a bus, lawyers wrote to editors asking them to respect her privacy.
When rumours of an engagement reached a peak around the time of Kate's 25th birthday in January 2007, a media scrum took up residence outside her Chelsea home.
Wills: "This may be the most nervous I have been"
By Robert Johnson
Addressing a small group of journalists at Kensington Palace before the screening of the new "official" documentary about his late mother, the Duke of Cambridge rubbed his hands together and looked anxious.
Dressed casually in a sports jacket and open-neck shirt, the future King's first words broke the tension in the room and got a laugh, too. "This is possibly the most nervous I've ever been," he said.
William is a smooth operator. When it comes to dealing with the media, he often uses self-deprecation as a tool. It's a device his father uses too.
He went on: "It's one thing doing a documentary like this, it's another thing standing in front of all you guys, explaining about something very personal. Nonetheless l think it is important that I am here to do that."
William added: "We haven't spoken so publicly about her before and we felt it was the right time to do it. We won't be doing this again."
That, I very much doubt.
For decades I have watched William's relationship with the press - I even conducted his first newspaper "interview".
I was allowed to fire a couple of pre-cleared questions to the young Prince during a ski holiday in Klosters in the mid-1990s.
He was holidaying with his father - by then separated from Diana - and brother Harry.
Cousins Beatrice and Eugenie were at the ski resort at the same time with Sarah Ferguson.
Even then William was painfully diplomatic.
When I asked who was the better skier between him and his younger brother (fearless dare-devil Harry was, by miles) William squinted in the sunshine, gestured towards his cousins and tactfully swerved the question - and the headline that would accompany the answer.
Diana's death has seriously damaged his relationship with the media, possibly permanently. As a result, William is still conscious not to give too much of himself away.
It may be a wise path. After all, manipulating the press - even in the courageous way she discussed her marriage problems in that legendary BBC Panorama interview - was something that left his mother exposed.
"This is the first time we've ever spoken about her as a mother and her legacy. It's still raw..."
By Nick Craven
Campaigner: Diana visits a minefield in Angola in 1997. Photo / Getty Images
Diana had something of a penchant for dressing her sons in strange clothes - a trait Harry now ascribes to her wicked sense of humour.
"I genuinely think she got satisfaction out of dressing me and William in the most bizarre outfits - normally matching," he says.
"It was weird shorts, and little shiny shoes with the old clip-on. Looking back at the photos, it just makes me laugh - I just think: 'How could you do that to us?'"
He says the fashion choices are the "one thing I would love to ask her [about] now," and admits he only began to object after William did.
"We got to the age when William would turn round and go, 'Oh, this is ridiculous, you know, I'm the older brother, why do I have to be dressed the same as him?' And I'm sitting there going, 'Hang on a second, if you're going to dress differently I'm not going to be the only person dressed like this; this is just ridiculous!'"
"So I like to think she had great fun in dressing us up. I'm sure that wasn't it, but I sure as hell am going to dress my kids the same way!"
Judging by George and Charlotte's outfits, it seems William feels the same...
