In 1991, writer Andrew Morton was granted unprecedented access to Princess Diana through tapes secreted out of Kensington Palace by a mutual friend. These interviews — requiring Morton to submit written questions that the friend would deliver to Diana — led to his incredible book, “Diana: Her True Story,” which revealed Prince (now King) Charles’s affair with future wife, Camilla Parker Bowles, as well as Diana’s eating disorder and suicide attempts. As Morton wrote for The Post in 2017, “I think she wanted to give the world an insight into her life, which was, in her opinion, a lie.” Now, as the tapes are being reviewed for a new documentary, Morton looks back on his historic experience.

For a moment it is just like entering a time machine.

Punch in the code for the fire-proof safe at a secret location in central London, take out the fragile cassette and carefully slot it into the tape player.

Press “Play” and that familiar voice, sometimes laughing, sometimes whispering, floods the silence. It is Diana, Princess of Wales, and for a moment there is such an immediate intimacy about her presence that the hairs stand up on the back of one’s neck.

It is like she is still alive, the years rolling back as she talks candidly about her life, loves and ambitions.

For the last 30 or so years the tapes have remained locked away, used sparingly in occasional TV documentaries.

Now, award-winning producer Tom Jennings, who has recently broadcast a film about King Charles, is combing through the tapes to tell her story from her point of view.

Provisionally titled “Diana: The Rest of Her Story,” Jennings’ project delves into her childhood, her school days and her family fights.

This important new documentary will give fresh insight into the life and times of a woman who dramatically changed and challenged the royal family.

As Jennings observed: “To hear Diana’s actual voice, walking the audience through her life allows viewers to see the tapestry of Diana’s world gradually appear. It is the closest thing we will have as we try to understand who she was as a member of the royal family, a mother and a humanitarian. She was a woman who stood up for herself and demanded her story be told.”

I vividly remember hearing the first tape she made. It was played for me in the incongruous surroundings of a blue-collar diner on the outskirts of London in May 1991.

A mutual friend, James Colthurst, acted as the go-between.

After I wrote out questions for Diana, Colthurst journeyed to Kensington Palace where the tapings took place.

There were six taped interviews that continued into the summer and autumn of that year.

Colthurst vividly remembers that first session. “We sat in her sitting room. Diana was dressed quite casually in jeans and a blue shirt. Before we began she took the phone of the hook and closed the door. Whenever we were interrupted by someone knocking, she removed the body microphone and hid it in the cushions.”

I was summoned to meet Colthurst one Saturday morning that fateful spring..To be honest I wasn’t expecting much, maybe a few remarks about her charities and humanitarian mission.

I couldn’t have been more wrong.

Diana talked about her desperate cries for help, her eating disorder and a woman I had never heard of, called Camilla.

It was apparent that the fairy tale marriage of the future king and queen — Charles and Diana — was an illusion.

During the tapings she was happy and amused when reflecting on her schooldays, laughing as she described winning the prize for having the best kept guinea pig.

Then her sunny mood was punctured as she recalled the bleak emotional landscape of her younger years and the trauma of her parents’ divorce.

She doesn’t hold anything back — expressing the real anger she felt towards her stepmother, Countess Raine Spencer, and the damage she felt Raine had wrought on her family and the family home, Althorp House.

“I was so angry,” Diana said.“I said, ‘I hate you so much. If you only knew how much we all hated you for what you’ve done. You ruined the house. You spent daddy’s money. I have said everything I possibly could.’ Raine said, ‘You have no idea how much pain your mother put your father through.’”

The revived tapes make clear, too, that, behind the public smiles, this was a family at war.

At Prince Harry’s christening for instance, Diana’s mother took Charles to task for wishing that his second child had been a girl.

It was a crass remark — especially as Frances Shand Kydd had given birth to a baby boy, John, who died hours later.

Diana recalled: “Mummy snapped his head off “and said, ‘You should realize how lucky you are to have a child that’s normal.’”

“Ever since that day, the shutters have come down,” she added of Charles. “That’s what he does when he gets somebody answering back at him.”

Given the passage of the decades, what is truly remarkable is Diana’s profound and uncanny sense of destiny.

As the documentary will explore, she knew in her heart that she would never become queen and that it was her destiny to travel along a path where the monarchy was secondary to her true vocation.

This is what she had to say about her future long before her separation and divorce from Charles.

“I am performing a duty as the Princess of Wales as my time is allocated. And if I go somewhere else, I go somewhere else. If life changes, it changes, but at least when I finish, as I see it, my 12 to 15 years as Princess of Wales … ” Diana said. “I don’t see it any longer, funnily enough.”

Sentences like these leave the scalp tingling, especially when you know the tragic end of her story.

For me, the most poignant moment of those conversations was when she confessed that she nursed a simple ambition: to walk along the boulevards and streets of Paris without being recognized. “Such a thrill,” she said.

Six years later, in August 1997, she died in a high-speed car crash in that city of dreams.

Recently the tapes have become even more important in describing and explaining Diana’s life.

The only other interview of note that she ever gave was to BBC reporter Martin Bashir.

During that now infamous conversation, Diana skewered her then love rival — Camilla Parker Bowles, now Queen Camilla — when she said: “There were three of us in this marriage so it was a bit crowded.”

in 2020, a former British supreme court judge found that Bashir had effectively “conned” Diana into giving him an interview by faking documents that suggested MI5, Britain’s domestic spies, had her under surveillance.

A furious Prince William described the Bashir interview as a “major contribution in making my parents’ relationship worse” and demanded it be banned from future airings. The BBC duly obliged.

What makes the tapes given to me so authentic is that the interviews took place over a period of months and were undertaken in the relaxed environment of her sitting room — the princess literally chatting to an old chum.

No cameras, bright lights or shiny shoed broadcasters confronted her.

While the tapes formed the spine of my resulting best seller, “Diana: Her True Story,” her friends and family also spoke out in her support.

They described her childhood, her school days and her romance with then Prince Charles — as well as the changes in her once she joined the royal family.

Their testimony added weight to her own memories and ambitions.

It was apparent during our remote conversations that her primary focus was on her boys: William and Harry, she admitted, were her “obsession.”

She never for a second considered that they would fall out so dramatically.

Indeed, Diana saw Harry as William’s wing man, not a hit man.

Nor did she think that Camilla would one day become queen.

Her strategy was for Charles to go off to live in Italy and spend his days farming and painting.

She felt that the crown should skip a generation and go straight to William.

As she said on the tapes: “If I was to write my own script, I’d say that I would hope that my husband would go off, go away with his lady and sort that out and leave me and the children to carry the Wales name through to the time William ascends the throne. And I’d be behind them all the way.

“Harry’s a ‘backup’ in the nicest possible way. William is going to be in his position much earlier than people think now.”

With King Charles now in his 75th year and undertaking more engagements in his first year as monarch than Queen Elizabeth II did when she was only 25, his early departure seems unlikely.

Who knows though.These tapes, now a unique fragment of royal history, are unnerving, documents which
describe a life foretold.

As Diana told Colthurst during one session: “From day one, I always knew I would never be the next Queen. No one said that to me — I just knew it.”

On paper these sentiments are fascinating, but hearing her voice say them makes a dramatic impact that stays with you.

Eerie, intriguing, and compelling, the Diana tapes have a potency that the passage of the years has only magnified. Welcome to Diana’s time machine.

Andrew Morton’s “Diana: Her True Story — In Her Own Words”  and his latest, “The Queen: Her Life,” are available from Amazon.com.