DNA tests on blood samples appear to prove the driver of Princess Diana's car was drunk on the night of her fatal accident, the BBC has learned.
The tests indicate original post-mortem samples, which showed Henri Paul was three times over the French drink-drive limit, were accurate and were his.
BBC Two programme The Conspiracy Files was told the French authorities carried out the tests within the past year.
Conspiracy theorists have claimed that Mr Paul's blood samples were swapped.
Some suggest this was done to portray him as drunk in an elaborate cover-up of a secret service plot to murder Princess Diana.
But the DNA tests would appear to prove that the driver's blood samples could not have been switched.
The new evidence has come to light days before the publication of Lord Stevens' report into Princess Diana's death in Paris in 1997, which is expected to conclude it was an accident.
Meanwhile, a poll for the BBC suggests that more than three in 10 people in the UK do not believe it was.
A source close to the French authorities told the programme the DNA tests were carried out in France within the past year.
A DNA profile was taken from Mr Paul's blood samples and compared with his parents' DNA. They matched.
Lord Stevens' inquiry team has pledged to investigate the many conspiracy theories that surround the deaths of Princess Diana, 36, her companion Dodi Al Fayed, 42, and Mr Paul, the driver of the Mercedes that crashed in August 1997.
The Alma Tunnel crash investigation was the biggest in French history and was carried out by the country's top police force, the Criminal Brigade. After two years it concluded Princess Diana's death was a tragic accident.
Mr Paul was held to blame for the crash and found to be drunk and driving at excessive speed.
However, conspiracy theories have gripped the public imagination as a result of doubts raised about the French crash investigation.
Sceptics of the official account question why a crucial witness, the driver of a second car involved in the crash, has never been identified and how it took nearly two hours to get Princess Diana to a hospital just four miles away.
Royal biographer Nicholas Davies tells the programme: "The more we are left with so many unanswered questions, the more we are left saying there is only one answer to this - that she was taken out."
Former royal correspondent Noel Botham claims the Princess sensed danger: "Diana was fearful of her own death for the last few years of her life. Right up to the moment of the accident she believed somebody was out there to get her and she was right."
Mohamed Al Fayed maintains his son Dodi and Princess Diana told him on the night they died of their plans to get married, and also that Princess Diana told him she was pregnant.
This CCTV image is one of the last ever taken of the princess
He believes this provided the motive for the British Establishment to organise a secret service plot to murder the princess.
Her close friend Rosa Monckton was on holiday with her three weeks before she died.
Ms Monckton maintains that the ring Dodi had bought for Princess Diana was not an engagement ring.
"I know there is a ring... she said it's going very firmly on my right hand. So no, she wasn't contemplating marriage with him."
Ten days before she died Princess Diana visited a London clinic for treatment of pre-menstrual tension.
Checks carried out by her consultant, Dr Lily Hua, confirmed that Princess Diana was not pregnant.
Ms Monckton also tells the programme she knows that "there's no way she could have been pregnant".
Professor Andre Lienhart reviewed the emergency services' response for the French investigation.
In his first interview he tells The Conspiracy Files he saw copies of the post-mortem examination carried out on Princess Diana in England 24 hours after the crash.
"The autopsy," he says, "showed that she was not pregnant.
"I don't think it's possible to stop rumours - what is clear from all the medical staff and paramedics working that night, is that they wanted to save the person in front of them. That is absolutely indisputable."
And Professor Lienhart adds there is one crucial fact people need to know: "What is certain is that she was not wearing a seatbelt and this made things worse.
"We would like to think that if she had been wearing a seatbelt, we'd been able to save her."
The investigation into the crash took two years to complete
This week Lord Stevens will make his judgement on the events of that night. It is widely expected that he will conclude the crash was an accident. The available evidence suggests Mr Paul was drunk at the wheel, with a potent mix of alcohol and drugs in his system, and driving too fast for the road.
The Metropolitan Police inquiry is likely to support the French police's view that the chasing paparazzi were an important influence on the speed of the Mercedes.
Martine Monteil, the head of the French Judicial Police who investigated the crash, tells The Conspiracy Files: "There was a horde of photographers who were following the couple and they were very close to the Mercedes when the accident happened.
"Obviously this causes annoyance and stress. But it is not the only explanation. The driver also lost control of the car, that's obvious."
How Diana Died: The Conspiracy Files was broadcast on BBC Two on Sunday 10 December at 2100 GMT.