ANDREW Morton knew, as he was writing his seminal book on Diana the Princess of Wales back in 1992, that he was crafting a piece of history.
The book Diana: Her True Story blew the lid off Diana's unhappy marriage to Prince Charles, his closeness to Camilla Parker Bowles, Diana's eating disorders and her suicide attempts.
It shook the royal household to its core, but what nobody knew at the time was that the book was based on taped interviews with the Princess of Wales herself, conducted by a mutual friend who then smuggled the tapes out of Kensington Palace and into Morton's hands.
That information only came to light after her death in a car crash in Paris, 20 years ago next month.
Now, 25 years after it was first published, Morton has revised his book, including more details from the taped interviews, and revealing the restrictions he was under as he wrote what turned out to be the closest thing ever published to Diana's own biography.
"I realised it was a historic document I was producing,'' Morton said this week from his home in London, where he has just completed his next book on another complex female character, Wallis Simpson, the divorcee that King Edward abdicated the throne for.
"It was a piece of history.
"It was deemed to be unofficial but when you look at it, it's actually an official biography.''
Morton said if Diana was alive today, the extent of her involvement would still not be known. The book would forever have referred to information coming from those close to Diana - when in fact it came from the Princess herself.
"It was a thin veil even at the time,'' Morton said of his efforts to keep Diana's involvement in the book under wraps.
"But it gave her some wriggle room in terms of determining her future.''
The book, and Diana's bombshell Panorama TV interview three years later, changed forever the public view of the royal family and damaged the reputations of Prince Charles and his now wife Camilla so badly they have never fully recovered.
The tapes were recorded throughout 1991 and 1992, when the Wales' marriages was in its death throes and Diana was desperate to break out of the grasp of "the Firm'' - the royal family and its courtiers who she had never felt supported or welcomed her.
The book was not a perfect record of events at the time - libel lawyers refused to allow Morton to print that Charles, the heir to the throne, was having an extramarital affair with Camilla, so their relationship is referred to in the book as a "secret friendship.''
And Diana never mentioned that she herself had embarked on secret affairs, and was in the throes of falling for wealthy art dealer Oliver Hoare at the time she revealing damaging details about Charles and Camilla's illicit affair.
But it gave a warts-and- all look at the dysfunctional state of Wales' marriage and showed how deeply unhappy and emotionally unwell Diana was.
Morton, now 63, said the way the book had come about, through third-party interviews, was "absolutely unique.''
Diana herself chose Morton as the person she wanted to tell her side of the story and he obtained the information through the help of a third party, Dr James Colhurst, a friend of Morton's and an old and trusted friend of Diana's.
Colhurst would go to Kensington Palace, read Morton's questions to Diana, record her answers, and give the tapes to Morton.
As a royal watcher, who worked as a freelance author and was not associated with any of the major newspapers, Morton knew Diana but only from a distance.
"It was always fairly superficial, as you do at a cocktail reception,'' he said.
"That's why I was so astonished at being asked to write the book.''
He believed Diana wanted to deal with someone not connected with the newspapers, who had close ties to Buckingham Palace and the royal court.
"It was about control, taking control of her life,'' he said.
Morton also knew the book, and Diana, would be still front of mind for many people decades later. "Quite frankly, I did (know),'' he said.
"She is a fascinating character and no-one has been able to replace her.
"Think about how hysterical people get about everything to do with Diana. Look at the really vitriolic comments on stories about her. There's no balance on the dial, even 20 years after her death.
"She was a lightning rod, in many ways.''
Morton continues to watch the royal family, and noted, although disagreed with, press reports this month comparing Prince William's wife, Catherine, to Diana.
The comparisons were meant as a compliment, after thousands turned out to see Catherine and press flowers and gifts upon her at public events in Germany and Poland.
"Poor old Kate has always been compared with Diana,'' he said.
"She's very different. Her and Prince William act much more as a team (that Charles and Diana), and she married into the royal family a decade older than Diana did.
"She's effectively lived with William for seven or eight years and she was introduced very gently into the royal family, unlike Diana who was thrown in the deep end and expected to swim.''
Morton believes the new approach with Catherine is as a result of the changes Diana forced after staking out her position through his book and her Panorama interview.
"These are the lessons the royal family learned,'' he said.
The updated book has a slightly different headline, with "in her own words'' added to the cover to make Diana's involvement in the story clear.
"I have always said Diana: Her True Story is not a balanced portrait in that there is no Prince Charles voice,'' Morton said.
He said he was "totally blindsided'' to later discover Diana's several affairs.
"I only worked out later working backwards through the timings that she was getting involved with Oliver Hoare while she was sitting there, talking about her life,'' he said.
"I had my hands full I suppose proving the relationship between Camilla Parker Bowles and Prince Charles, and the eating disorders and the suicide attempts.
"I never said they (Charles and Camilla) had an affair, as the libel lawyer said to me: 'were you in the room at the time'?
"He was the one who advised me to use the phrase "secret friendship.''
Morton said his co-conspirator in the book, Colhurst, continued to practise as a homeopathic doctor in Berkshire, where he had worked for more than 20 years.
He recalled writing topics down for Colhurst under topic, such as "Charles'', or "wedding day'' and waiting to see what would come back after Colhurst had sat down with Diana at Kensington Palace.
They had been friends for many years and his presence at Kensington Palace did not raise suspicions, as Morton's would have done.
"James was kind of helping a friend (Diana) out of a tight spot,'' he said.
"There was a kind of casualness about the whole thing.
"He was very protective of her, very concerned to look after her welfare.
"But he also understood that she was at the end of her tether.''
He said the tapes and the information they provided "very much depended on the mood she was in.''
"But she often talked about the past, her childhood was pretty difficult as well.''