When Princess Diana died in 1997, her "box of secrets", containing 10 never-before-seen video tapes and audio cassettes she covertly recorded, disappeared from Kensington Palace – and have never been recovered.
In Diana: Case Solved, authors Dylan Howard and former detective Colin McLaren claim the lost tapes contained a "record of her knowledge of scandals that could finish Britain's monarchy" and that she "stashed them in a box called the 'Crown Jewels'".
They were kept under lock and key in her Kensington Palace apartment along with a signet ring belonging to her former lover James Hewitt and photographs that show her ex-husband Prince Charles romping naked with a male lover, their informants claim.
"As far-fetched as it sounds, the claims were central to the 2008 inquest into her death when her sister, Lady Sarah McCorquodale, strenuously denied she destroyed the box of secrets," the authors say.
"Instead Lady Sarah insisted she gave them to [Diana's butler] Paul Burrell for safekeeping, adding pointedly, 'I trusted him then.' Paul maintains, however, that the papers were taken to the Spencer family seat at Althorp, where they are hidden."
When questioned under oath about handing over to the butler material considered so sensitive it was compared to the contents of the mythical Pandora's box, Lady Sarah insisted that is exactly what happened.
She confirmed that six months after Diana's death she and Paul "found the key to the mahogany box hidden inside the cover of a tennis racket" and confirmed that when they opened the box they found "highly sensitive" material.
Diana famously recorded 16 videos with her voice coach Peter Settelen in 1993, just months after her separation from Charles.
Six of them were used for a controversial documentary that aired in Britain two years ago, but the remaining 10 have never been seen. More worrying for the royal family is that, according to this new investigation into her death, she also recorded 12 tapes detailing her most intimate secrets.
"The royal family was said to be 'petrified' Paul would reveal the allegations squirrelled away on tape recordings," the authors claim.
"The royals knew she had been collecting information on them for years, and wanted her out of the way. They couldn't have had their trail of affairs and seedy secrets coming out in the open."
The secret stash of recordings and photographs were among the items Paul took to the attic of his home after Diana's death – which is why the police raided his house on January 18, 2001, and also the reason his theft trial was aborted at the last minute.
"As everyone knows, the Queen herself stepped in at the eleventh hour to state she had suddenly recalled giving Burrell permission to take keepsakes from Diana's apartment – something Burrell had claimed all along," the book says.
"It was only an intervention of this magnitude that could stop the world hearing the butler reveal exactly what was among the items he had lifted."
Paul was later summoned to a three-hour meeting with the Queen, during which, he says, she chillingly told him that his life may be in danger from knowing too much about the royal family.
The authors claim one of the most damaging secrets in Diana's Crown Jewels was "photographic evidence Charles was gay", and a recording of an interview she did with Charles' former manservant George Smith, who was embroiled in a tabloid scandal after claiming he was raped by Michael Fawcett, Charles' former valet.
READ MORE: The 50 biggest royal scandals of all time.
According to George, Michael was Charles' gay lover and George claimed he walked in on the pair having sex.
WATCH: Princess Diana and Prince Charles exchange wedding vows in 1981. Post continues after video...
"The scandal would never have emerged if Diana had not sat at the end of Smith's bed after he was hospitalised following the alleged rape – and recorded his damning attack on Charles," the book says.
"Understanding that knowledge was power, the princess – paranoid even in 1990 about her safety – kept that tape with her other cassettes containing royal secrets, locked away. Among the treasure trove were vicious letters sent to her by her father-in-law Prince Philip, which have also never been seen."
Police dismissed George's rape allegations against Michael, while his claims of a sexual encounter between Charles and Michael were described as "ludicrous and risible" by the Prince's private secretary, Sir Michael Peat.
"It was her stash of secrets that drove the Queen and Philip to stop the Burrell trial," the book claims.
"The royal family was said to be 'petrified' Burrell would reveal the allegations squirreled away on the tape recording... insiders believe Diana may have been killed to cover up the secrets they never thought would emerge."
One aide, who spoke to the authors on the condition of anonymity, claims the royals knew Diana had been collecting information on them for years.
"What they never betted on was that she had taped their most intimate secrets – and had photographic evidence Charles was gay."