The grim milestone of the 25th anniversary of the death of Diana, Princess of Wales, was marked this week. The princess died tragically in a car crash in Paris on August 31, 1997. She and her boyfriend Dodi Fayed were travelling from the Ritz hotel when their driver, Henri Paul, lost control of the vehicle at a high speed and crashed. Diana, Dodi and Mr Paul all died from injuries sustained in the crash; Dodi’s bodyguard, Trevor Rees-Jones, was the only survivor.

Investigating Diana: Death in Paris delves into the two police investigations into Diana’s death — the French police’s initial probe in 1999 and the Metropolitan Police’s inquiry nine years later.

In both inquiries, investigators faced the additional challenge of dismantling the conspiracy theories surrounding the fatal incident.

In 2003, a year before the British inquiry was opened, a chilling letter written by Diana was published in a national newspaper.

The Daily Mirror published a photograph of the note, which read: “I am sitting here at my desk today in October, longing for someone to hug me and encourage me to keep strong and hold my head high.

“This particular phase in my life is the most dangerous — ‘X’ is planning ‘an accident’ in my car, brake failure and serious head injury in order to make the path clear for Charles to marry…

“I have been battered, bruised and abused mentally by a system for 15 years now, but I feel no resentment … I am strong inside and maybe that is a problem for my enemies.

“Thank you Charles for putting me through such hell and for giving me the opportunity to learn from the cruel things you have done to me.”

The letter was addressed to Paul Burrell, the former royal butler.

He revealed he had kept the letter private through six years of police investigations and media speculation.

It came around the same time he released his memoir, ‘A Royal Duty’, which followed his career as a member of the Royal staff.

The letter was also included in his book, but the name of the person allegedly conspiring to kill the princess was blacked out.

It was later revealed Diana had written “my husband”.

Paul Burrell appeared on the recent Channel 4 documentary and said: “All I can tell you is behind closed doors I was looking after my friend. She was called Diana.

“I understood her fight and her struggles. And I knew where all the enemies were. And they were all coming for us at the same time…They were all against us.”

Mr Burrell then listed the Queen’s household, Prince Philip’s household and Prince Charles’ household as the “enemies”.

He continued: “The police wanted to see the note, which was a private note, but I thought it was of national importance and they should see it, to know how vulnerable she was, how frightened she was and how marginalised she was…

“[Charles] couldn’t possibly be responsible for the murder of his former wife. That would be the end of the monarchy.”

David Douglas, an investigating officer in Operation Paget, said: “What that headline is saying, and that note is saying, is that the Princess of Wales, in 1995, feared there might be a plan to get rid of her in an accident.

“You can’t dismiss that because two years later, she died in a car accident.”

He added: “‘My husband is planning to get rid of me’. That’s what Diana wrote.

“So, clearly, in her mind, at the end of October 1995, for whatever reason, this is what she wrote down as her thoughts.”

Duncan Larcombe, a royal journalist and author, told the Channel 4 documentary that the letter “was a complete game changer, another missing piece of the jigsaw.”

He continued: “‘Diana’s fears that she’d be murdered,’ that fits in with what [Mohamed] Al-Fayed said. It also fits in with the large flash.

“Suddenly, you’re joining the dots.”

However, no evidence surrounding the crash suggested Charles, or any member of the Royal Family, were invovled in the incident.

And a friend of the late princess has claimed she was “prone” to this sort of paranoia.

In 2019, Richard Kay, Daily Mail’s editor-at-large and an old friend of the princess, told the Fatal Voyage: Diana Case Solve’ podcast series: “She was prone to these sort of fears, that something might happen to her.

“People convinced her that she was somehow some sort of present danger to the Royal Family and that she should therefore be very careful.”

More recently, Diana’s eldest son William said his mother was plagued by “fear, paranoia and isolation” in the years before her death.

He claimed Martin Bashir, who used “deceitful behaviour” to secure his bombshell interview with the princess, contributed to her paranoia.

Both the initial French investigation in 1999 and Operation Paget, which came to a close in 2008, ruled the tragic car crash as an accident.

However, conspiracy theorists still speculated that Diana and Dodi were deliberately killed, with some claiming Mr Paul was in on the plot and others suggesting another vehicle had purposefully forced the black Mercedes to swerve and crash.

A prominent believer in these claims was Dodi’s father, Mohamed Al-Fayed, a successful businessman who owns the Ritz hotel and formerly Harrods.

On several occasions, Mr Al-Fayed claimed senior members of the Firm were behind the fatal crash, particularly putting blame on Prince Philip.

He also alleged the Firm was not happy about Diana and Dodi’s relationship, which he claimed had become more serious before the incident.

Mr Al-Fayed alleged Diana and Dodi were engaged and even claimed the princess was pregnant.

However, while Dodi was caught on CCTV purchasing a ring, there was no indication the couple had become engaged.

Furthermore, no evidence suggested that Diana was pregnant.

French investigators deemed Henri Paul as solely responsible, while the British police blamed the negligent driving of Mr Paul as well as the pursuing paparazzi vehicles.