Four days after the inquest into Diana’s death was opened, The Times has discovered that there are high-level concerns over the forensic evidence at the heart of France’s investigation.

The French authorities have failed to carry out DNA tests to prove that the specimen belonged to the chauffeur Henri Paul, The Times has learnt.

This threatens the conclusion of the French authorities that Diana was killed by a driver high on alcohol and prescription drugs who lost control of a car while speeding.

The French inquiry into Diana’s death in a Mercedes in a Paris road tunnel on August 31, 1997 has been carefully monitored by United Kingdom diplomats, Whitehall and police. France has been resisting pressure from M Paul’s family, and advice from UK officials, to carry out DNA tests which would finally prove that the blood belonged to M Paul.

The source of the British suspicion is that the sample contains an extraordinarily-high level of carbon monoxide, so much so that the chauffeur would have struggled to walk, let alone drive a car.

It is now feared by the authorities in London that an innocent mix-up in the laboratory or morgue may have led to the wrong sample being tested.

One possibility is that the sample comes from the corpse of somebody poisoned by carbon monoxide, the deadly gas found in household fires and car exhausts. The blood specimen is at the centre of the official French explanation of the deaths of Diana, her boyfriend Dodi Fayed, and M Paul.

The examining magistrates Hervé Stephan and Marie-Christine Devidal said that the three died as the result of an accident, rather than a deliberate act. This was because “the driver of the vehicle was drunk and under the effect of medicine incompatible with alcohol, a state which did not enable him to maintain control of his vehicle while driving at high speed on a difficult part of the road, and also having to avoid a vehicle travelling in the same direction at a slower speed”.

The blood, purporting to come from M Paul, indicated he was three times over the French drink-drive limit, and twice over the British.

If the blood sample cannot be positively connected to the chauffeur, there is still evidence that his driving may have been to blame for the deaths. Scotland Yard sources have indicated that they have a high regard for the quality of the French road traffic accident investigation, which they hail as “exemplary”.

The Mercedes S280 was travelling at a speed somewhere between 74mph and 97mph when it entered the Pont d’Alma tunnel, and at between 59mph and 68mph when it hit a pillar. The finding that M Paul had a high level of alcohol in his blood was first made by the Paris prosecutor’s office on September 1, the day after the fatal crash.

Lawyers for Mohamed Al Fayed, Dodi’s father, sought an independent analysis of the blood samples but Judge Stephan refused. The judge ordered new tests in order to counter any future challenges. Blood, hair and bone marrow was drawn and examined on September 4 with the whole procedure recorded on video.

Small traces of tiaprise — used to treat pain or aggression, often in chronic alcoholics — were found. So was a therapeutic dose of fluoxetine, the key active ingredient in the anti-depressant Prozac. “Care in the use of these medicines is habitually recommended to drivers,” the public prosecutor’s office said. Analysis of protein transfer in M Paul’s blood produced results “compatible . . . with a chronic alcoholism over the course of at least a week”, the office said. One of the samples showed 20.7 per cent of the blood had combined with carbon monoxide, an unusually-high level. Mr Al Fayed has long claimed that the blood samples were swapped by British and French intelligence agents to cover up murder.

In August 2002, the chauffeur’s family filed a complaint of “falsification of expert evidence”, without naming a defendant in a Paris court. The aim was to force the authorities to hand over blood samples for DNA tests. Their suit has been rejected as unfounded. Mr Al Fayed’s lawyers have raised questions about the constant refusal by the authorities to grant access to the samples or to M Paul’s body.

Jean-Claude Mules, a police inspector who played a central role in the investigation, said: “There was no error over the blood. We are very serious people and no errors are allowed.”

However Jean Paul, the father of the late Ritz Hotel security official, said: “We remain absolutely convinced that our son had not been drinking.”

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