Failing to track down a white Fiat Uno that may have been in the tunnel on the night that Diana, Princess of Wales was killed in a car crash is a lasting “frustration” of the French inquiry into her death, investigators have said.

Diana was 36 when she was killed in a car crash in Paris on August 31 1997 and her death has continued to attract conspiracy theories over the decades.

Detectives from the 1997 French Brigade Criminelle tell a Channel 4 documentary, Investigating Diana: Death In Paris, that the driver could have helped to try to explain the movements of the black Mercedes she was travelling in.

Martine Monteil, head of the Brigade Criminelle, said: “The whole of the world has struggled to accept that the Princess of Wales died in a mundane accident.”

She added: “I have frustration about the Fiat Uno because I like a well-finished business.

“For sure, it’s out there. Unfortunately we don’t have it.

“But you know the driver of the Fiat Uno, he’s not the real culprit.

“He’s driving along quietly and then a Mercedes arrives at high speed and bumps into him. The responsibility remains with the Mercedes.”

The black Mercedes that Diana and her partner Dodi Al Fayed were travelling in was being pursued by the paparazzi after they left the Ritz Hotel in Paris.

It crashed in the Pont de l’Alma tunnel.

Mr Fayed, 41, son of Harrods owner Mohamed Al Fayed, and driver Henri Paul died.

Bodyguard Trevor Rees-Jones survived.

A couple at the scene saw a white Fiat Uno coming out of the tunnel with a driver who was focused on his mirrors, the investigators said.

The couple described a brown skinned man with short hair and a dog wearing a muzzle being in the car.

Traces of white paint were found on the black car and its tail light was broken.

Fabrice Cuvillier, of the Brigade Criminelle, said the Fiat Uno exists, telling the programme: “It’s not a hallucination. It’s not something we threw out to create a diversion. It exists.

“We did not find this Fiat.

“But it would be dishonest of me to look you in the eye and say ‘we didn’t let the Fiat Uno slip through’.

“I don’t think so, but I’m not 100% sure.”

Eric Gigou, of the Brigade Criminelle, said “we did everything we could to understand what happened” and more than 1,000 people were interviewed in the investigation by the French authorities.

He told the documentary: “In my mind the only door that remains open is the testimony of the driver of the Fiat Uno.”

Diana’s death became the subject of many conspiracy theories, from claims by Dodi’s father that the couple were murdered in a plot hatched by MI6 on the orders of the Duke of Edinburgh, to suggestions Diana was pregnant.

Other theories centred on the lack of CCTV footage from the tunnel and the mysterious white Fiat Uno, which is said to have come into contact with the Mercedes, but which has never been traced.

Ten years after the crash, a high-profile £4.5 million inquest found that the princess was unlawfully killed because driver Henri Paul was drunk and driving too fast, the car was being chased by photographers and that Diana and Dodi might have survived had they been wearing seatbelts.