A TV documentary on Wednesday said Henri Paul's blood had an implausibly high level of carbon monoxide, twice that of a 40-a-day smoker.

M Paul: 'Happy before crash'

Now Nicholas Farrell who worked on the ITV documentary 'Diana: the Secrets behind the Crash', says the blood could have been switched.

He says in The Spectator magazine that Henri Paul was "seriously dosed up" in the high-speed car crash in a Paris road tunnel last August.

"Not only had he drunk more than three times the French drink-drive limit and taken antidepressants when he got behind the wheel of the Mercedes, he was also halfway dead from carbon monoxide poisoning for good measure," he said.

Mr Farrell discounted the theory that the high level of carbon monoxide could have been the result of a fault in M Paul's personal car since an MoT test showed nothing wrong.

Mr Farrell said: "What about the Mercedes itself? Again, virtually impossible. M Paul died instantly."

Friends of Diana attacked "deplorable" documentary

Another theory considered by Mr Farrell was that M Paul was attempting suicide since he was, after all, taking antidepressants, but video footage prior to the accident showed M Paul happy.

Mr Farrell added: "So if M Paul was not trying to kill himself there are only two alternatives: either the French doctors who conducted the autopsy got the wrong answers when they tested his blood, or else they tested the wrong blood.

"The implications of this are enormous. If it was not M Paul's blood which was tested, then it means we do not know if he was drunk and had been taking antidepressants."

"And if the wrong blood was tested, was it a genuine mistake by the doctors, or did someone, MI5 or whoever, switch the samples?

"I do not believe in conspiracy theories, but it makes me wonder."

The documentary, which had the endorsement of Mohammed Al Fayed, has been attacked by royal experts and friends of Diana.

All witnesses will attend the inquiry into the crash

Lord St John of Fawsley said: "I think these programmes are deplorable.

"It was in the worst possible taste and is extremely distressing for members of the royal family, especially Prince William and Prince Harry."

But Granada Television defended the programme, saying it was sensitive to the feelings of the two princes.

Another documentary, Dispatches on Channel 4 on Thursday, unearths a witness who claims to have heard Paul taunting photographers outside the Fayed-owned Ritz Hotel in Paris with the words: "Don't try to follow us, you'll never catch us."

Conspiracy theories

Dispatches presenter Martyn Gregory said: "The main problem with the conspiracy theories, many of which have been propagated by Mohammed Al Fayed, is that there has been no shred of credible evidence to support them."

A last bid to establish the full truth of the car crash which killed Diana, Princess of Wales, is being made in Paris on Friday amid the welter of conspiracy theories and alleged new evidence.

Crash inquiry judge Hervé Stephan has called all key witnesses together to thrash out inconsistencies in their evidence.