Diana Inquest: Chauffeur Henri Paul had 'mysteriously high' carbon monoxide levels before crash

The percentage of toxins in his system was so abnormally excessive that the Frenchman should have been suffering symptoms including severe headaches and nausea, the High Court was told.

Yet hours before his death, Mr Paul was captured on CCTV at the Ritz Hotel parking his car, walking around and even tying his shoelaces in apparent good health.

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Mohamed al Fayed, whose son Dodi died in the August 1997 crash along with Mr Paul and Diana, Princess of Wales, is convinced the samples were switched as part of a wide-ranging MI6 cover up.

He claims the couple were assassinated on the orders of the Duke of Edinburgh who disapproved of his former daughter-in-law's relationship with a Muslim.

Yesterday Richard Keen QC, the barrister representing Henri Paul's parents, suggested that the samples had been "spiked" in an attempt to "post humously murder" the Frenchman's reputation and portray him as unfit and a drunk.

Toxicologist Professor Robert Forrest, who has already suggested that Mr Paul may have drunk up to eight glasses of strong spirits in the hours before the crash, leaving him twice the UK legal drink drive limit, admitted that he could not find any logical explanation for the findings.

"One is left with either analytical error or a mystery," he told the jury.

"It's either conspiracy or cock-up."

According to tests carried out by two French medical experts hours after the Paris crash, the levels of carbon monoxide in Mr Paul's blood ranged between 12 and 21 per cent.

This compares to a normal reading of around two to four percent. Several possibilities were explored including the suggestion that the driver inhaled gas from the airbag of the Mercedes after impact.

This was discounted by Prof Forrest, however.

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The best suggestion that he could come up with yesterday was that the reading could have been connected to his heavy smoking.

He also pointed out that the machine used to measure blood gas was a "good piece of kit" but not one designed for post mortem analysis.

But Prof Forrest admitted that it was "very, very unlikely" Mr Paul could have realistically had such a high level of Carboxyhaemoglobin concentration in his blood and said there was no rational explanation for the result.

"I would say still that this is the one thing that worries me most about this investigation," he said.

"I still have not managed to achieve an intellectual resolution which totally satisfies me."

Earlier Prof Forrest told the jury that the amount of alcohol the driver had consumed in the hours before the crash had left him twice the UK drink drive limit and three times the French.

"The alcohol present in his blood at the time of death would, unequivocally, have adversely affected his ability to safely control a motor vehicle," he said.

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The jury was told that they would, in due course, hear evidence from Mr Paul's closest friend, Claude Garrec, that he was a "convivial, bon vivant" whose tolerance to alcohol was such that he would fail to show signs of being drunk.

Indeed, according to Mr Garrec, his old friend would 'regularly' drink and drive without any "noticeable effect".

Prof Forrest agreed that this was not uncommon in heavy drinkers such as Mr Paul, who was described in court yesterday as a "chronic user of excessive amounts of alcohol".

"When you have had a few drinks you can often drive a route that you known well without any adverse problems developing or being apparent. But if something unexpected happens, some unusual combination of events takes place, that is when the driver who has been drinking may well come to grief," he said.

The inquest will hear today (Wed) from bodyguard Trevor Rees, the only survivor of the crash.

Former paratrooper Mr Rees, who was working for Mr Al Fayed, broke every bone in his face and suffered terrible chest and head injuries.

In a written statement which was read to the court yesterday, his psychiatrist Dr Maurice Lipsedge warned that he may never remember the final moments of the doomed journey.

"In view of the time that has elapsed, the chances of him recovering his memory are very slight," he said.

Mr Lipsedge said he had three psychiatric counselling sessions with Mr Rees, the first just a month after the accident.

"He is an exceptionally strong person who has not suffered from depression or post-traumatic stress since the accident, " he said.

"[But] He sustained severe head injuries and it is quite common for this to cause major problems with memory.

"In this case, Trevor Rees-Jones remembers getting into the Mercedes in the Rue Cambon and the car driving off.

"He remembers nothing after that."