A British lawyer working on the case said: “It is scarcely believable that such crucial evidence could be lost just weeks before the inquest. This could create enormous problems for us.”

The dossier included 200 witness statements, files of photographs, detailed test results and other vital evidence gathered in one of the longest and most expensive investigations in French legal history.

A team of 30 senior police officers worked on the case to gather the evidence for the inquiry headed by French magistrate Judge Herve Stephan.

But it is not the first time the Diana dossier has apparently been tampered with. Last May photographs of the princess and Dodi taken at the crash scene – which were part of the official report – were found to be missing.

The legal source added: “To mislay one or two photographs is bad enough but can be explained away. It would be ridiculous, however, to suggest that a 6,000-page dossier containing all the evidence in such a high-profile case has been lost.”

The disappearance also casts doubts over whether all the evidence gathered by the French authorities was handed to former Scotland Yard chief Lord Stevens. It was his investigation that concluded Diana died in an accident.

Last night it was not clear how long the dossier – found to be missing by French lawyer Jean-Louis Pelletier – had been missing.

Mr Pelletier is representing French paparazzo Fabrice Chassery – one of the photographers allegedly pursuing Dodi and Diana when their chauffeur-driven Mercedes careered into a concrete pillar in a Paris tunnel.

“I went to the court building to examine the files but I was told that they were not there.

“I know files go missing occasionally but, bearing in mind the size and importance of this particular one, it is extraordinary,” he said.

“I went to every different part of the building, thinking perhaps it had been moved from the High court archives to the Criminal court or the Appeal court, but no one could find it.”

He added: “A search on the computer to try to locate it also revealed nothing. I am amazed that something like this could simply vanish.”

Mr Pelletier said he discovered that a photograph he was looking for had gone missing when he asked to view the file earlier this year.

He said: “It was odd. One week the photo was there and the next it was gone. It was a photo taken by my client Mr Chassery and at the time I believed it had been stolen. But now it is much worse. The entire dossier has gone missing.”

Mr Pelletier explained how he needed to see the dossier because his client had been facing manslaughter charges over the tragedy. He was later cleared but is now facing a private prosecution from Mr Al Fayed.

Copies of the dossier still exist. One was sent to Lord Stevens while another was handed to Lord Justice Scott Baker, the coroner who will preside over Diana’s inquest next month.

Scotland Yard detectives have at least one copy – but the files are not complete and evidence could be ruled as inadmissible in a court hearing if original documents are not presented.

Embarrassed French officials could face the prospect of asking the British to give back a copy of their own dossier.
But the legal source warned: “Photocopies simply aren’t good enough – we need the original documents.”

A spokesman for the Palais de Justice said last night: “Several requests have been made by those connected to the Diana crash investigation to view the dossier. We are endeavouring to re-locate it.”

Judge Stephan and his colleague, Judge Marie-Christine Devidal, found that French chauffeur Henri Paul, who also died in the crash, was mainly to blame for the shocking tragedy in the Alma underpass in August 1997.

Since then fresh evidence has cast major doubts over the findings, following claims that the authorities were involved in a comprehensive cover-up.

The startling discovery that such crucial primary evidence cannot now be accounted for is bound to further increase suspicions.