A new documentary has made a bold claim about the infamous letter written by Princess Diana. Photo / AP

A new documentary has made a shocking claim about a letter Princess Diana wrote in which she predicted her death.

The infamous letter penned by the Princess of Wales warned she could be killed in a staged car crash, with new claims insisting it was only passed to French investigators six years after her fatal collision.

The recently released film Investigating Diana: Death In Paris refers to the so-called Mishcon Note as it was written by the Princess of Wales' legal adviser in 1995, and is reportedly an account of what Diana said during a meeting with Victor Mishcon and her personal private secretary at the time.

Diana apparently alleged she had been told by a source there would be plans to "get rid of her" by April 1996, using the method of a car crash that would either leave her maimed or dead.

The Daily Mail has also claimed even though Mishcon handed his note to senior Metropolitan Police officers in 1997 when Diana died, it was not sent to French authorities probing her car crash in a Paris tunnel until six years later.

Diana's brothers and sisters only found out about the letter more than a decade after it was written, with Prince William and Harry also unaware of it for a long time, according to the newspaper.

The Investigating Diana film is a co-production between Channel 4 and Discovery Plus, billed as a "gripping and forensic police procedural" which will expose how "powerful individuals, the press and the internet created and fuelled conspiracy theories that overwhelmed facts and called into question the very nature of truth".

It will feature interviews with detectives from both British and French forces who investigated Diana's crash, with some of them speaking publicly for the first time.

Diana died 25 years ago aged 36 on August 31, 1997, alongside her partner and rumoured fiancé Dodi Al-Fayed, with their driver Henri Paul also killed.

Paul was ruled to be under the influence of alcohol and prescription drugs, with a lengthy 2008 inquest finding Diana was unlawfully killed as a result of "grossly negligent" driving.

It comes after the late Princess's former bodyguard boldly claimed she would still be alive today if he had been in the car with her on the night she died in a Paris car crash.

Lee Sansum told The Sun that security guards drew straws to accompany the royal's bodyguard Trevor Rees-Jones in the car on that fateful night in August 1997.

"It could have been me in that car," Sansum told the publication.

"We drew straws to see who would be accompanying Trevor that weekend. I pulled a match and it was a long one.

"When I learned [Diana and Dodi Al Fayed] were not wearing seatbelts in the crash I understood why they didn't survive. That shouldn't have happened.

"It was standard practice for the family to wear seatbelts. It was an order sent down from the boss, Dodi's dad Mohamed Al Fayed. Dodi, in particular, hated wearing seatbelts and I always insisted on it.

• It is not yet known when Investigating Diana: Death In Paris will be available to view in New Zealand.

- Additional reporting by NZ Herald