1) There was a blinding flash from an antipersonnel stunning device just before the crash. This lit up the tunnel, dazzling driver Henri Paul who lost control of the vehicle because of this.
This is unsubstantiated.
Richard Tomlinson, the former MI6 officer, has claimed that a similar method was part of a plan to assassinate the then Serbian leader Slobodan Milosevic in 1992. Tomlinson has also apparently claimed that Henry Paul was in the pay of the British secret service.
2) A Fiat Uno deliberately clipped Diana's Mercedes as it entered the Alma tunnel. It's alleged driver, paparazzo James Andanson, died two years later in a burned-out car.
The car has never been traced.
The French probe into the accident says the white Fiat Uno did not cause the crash.
3) After their marriage, Dodi and Diana were to live in the Villa Windsor, the former Paris mansion of the Duke and Duchess of Windsor, which was being leased to Mohammed al-Fayed, Dodi's father. The couple had actually been to the villa to see an interior decorator to prepare the house for them to move in to.
Dodi and Diana did take in a tour of the villa hours before the crash, apparently on orders from al-Fayed, however bodyguard Kez Wingfield, speaking to the News of the World, 28th August 2005, denied claims that there was an interior decorator with them with the intention to model the villa into a home for the couple. According to Wingfield they were in the house only 31 minutes, Diana was 'bored stiff'.
4) Pont d'Alma underpass is proven accident spot.
Computerized video simulations can reproduce the accident using the car's speed, its weight with four adults aboard, the condition of the road surface, the slope of the road, the weather, visibility and other traffic.
No driver, however skilled, would have been able to prevent the Mercedes crashing if it was traveling at that speed in those circumstances.
5) The Mercedes had been involved in a crash only nine months earlier.
It is true that the car had been involved in what has been described as a 'shunt' nine months earlier.
French experts from the Institut de Recherche Criminelle de la Gendarmerie Nationale, who examined the vehicle, have concluded the Mercedes had low mileage and had been in perfect mechanical and working order.
6) Diana was pregnant to Dodi Fayed. Supporters cite rumors that the princess had a pregnancy scan weeks before her death and point to photographs of her, taken around the same time, showing alleged slight swelling in her stomach.
The Metropolitan Police have dismissed the story.
Friends have dismissed the pregnancy claim as nonsense. Dr John Burton, the former royal coroner who was present during the princess's post-mortem, has also insisted that she was not pregnant.
Supporters of the theory say, however, that because the princess was embalmed the formaldehyde used in the process would have corrupted the results of a pregnancy test and given a false result.
7) Diana was planning an engagement to Dodi Fayed. The apparent evidence is a £130,000 diamond engagement ring said to have been bought for the princess at the Monte Carlo branch of Repossi (for collection in the Paris branch) when their yacht was moored off Monte Carlo. Security cameras show Dodi apparently examining a ring inside the Paris branch of Repossi hours before the fatal crash.
Bodyguard Kez Wingfield, as reported in the News of The World, 28th August 2005, says that no ring was purchased in Monte Carlo. Wingfield says that Diana and Dodi didn't leave his sight at any time.
Wingfield also claims that Dodi and Diana never exhibited any signs of romance, describing their relationship as like a brother and sister's.
8) One of the most widely propounded theories, supported by Mohammed al-Fayed, whose son Dodi, also died in the crash, is that the princess was the victim of a 'horrendous' murder organized by the British establishment, and possibly carried out by MI6 officers, brought about because of alarm in royal and government circles about Diana's relationship with his son, who was a Muslim.
Michael Burgess, The Observer 28th August 2005: "All the 'conspiracy theories' are mainly the product of Mohammed al-Fayed and a brilliant PR campaign run by former journalists and apparatchicks at Harrods."
"With a budget of more than £5 million, its central aim is to discredit the French investigation. Fayed has recently been distributing a video The Mystery of the Alma Tunnel, in which he says: 'I pledge my life to bring Prince Philip and his terrorist thugs to justice before the British people and the people of the world.'"
9) Ex-butler Paul Burrell has produced a document in which Diana claims Prince Charles was plotting to kill her in a car accident.
Michael Burgess, The Observer 28th August 2005: "French police contacts I spoke to said they were aware that Diana had often spoken of a threat to her life, but it did not affect the facts of the accident."
Martin Bentham: "The most senior French investigator dismissed this as 'absurd'."
10) Driver, Henri Paul, was an MI6 agent who sacrificed his life to remove Diana and Dodi.
Less easy to account for is the high level of carbon monoxide in Paul's blood. His parents claim that this could not have been present if he died instantly, as is officially believed, and that therefore his blood sample, which also contained the alcohol and drugs, had been swapped. (See 11)
11) Driver Henri Paul's blood alcohol level was very high, however we see him walking apparently sober in security camera footage in The Ritz only moments before the crash.
Advocates of this conspiracy theory claim, among other things, that the drink and drug tests carried out on Henri Paul, the chauffeur on the fatal journey, were falsified and that the findings of the original post-mortem, that he was three times over the drink drive limit and had taken medication incompatible with alcohol, were wrong.
"The analysis of his blood showed a concentration of alcohol at an illicit level," a statement said. The legal limit of alcohol in a driver's blood in France is 0.5 grams per liter of blood, the equivalent of two glasses of beer or one glass of wine. Prosecutors said Henri Paul's blood-alcohol level was 1.75 grams.
24 Sept 2006: Latest news from the investigation into the crash, points to the possibility that French officials could have mixed up the blood samples from Henri Paul with another person. This is after it is alleged that the French hospital mixed up Henri Paul and Dodi's bodies in the hours after the crash. If this is true then it brings into doubt the official explanation that it was a case of drunk driving.
5th October 2007: Video evidence submitted to the Diana Inquest shows Henri Paul perfectly sober tying his shoelaces on an hour before driving the car which crashed.
Controversial Chi Magazine front page: But is this a genuine picture or a faked montage?
Tragic Postscript: Diana and Dodi would both probably have survived if they were wearing seat belts.
20th August 2013: What is being described as a credible witness has come forward claiming that the SAS were involved in a conspiracy to kill Diana, according to various newspaper reports including The Express (source). After the authorities initially tried to claim that the source was a loose canon, if the report in The Express is right, the same person, known only as Soldier N, gave evidence against Sergeant Nightingale (a Court Marshal for having a gun in his possession) in a prominent recent conviction on behalf of the Crown prosecution. Those trying to claim that Soldier N is now a loose canon on the Diana claims will have to explain why they used his evidence when it suited them in the Nightingale conviction, but claim he has no credibility now. Awaiting developments.
7th April 2008: Verdict of the jury in the Coroner's inquest into the Deaths of Diana, Princess of Wales, and Mr Dodi Al fayed: "The verdict is unlawful killing, grossly negligent driving of the following vehicles and of the Mercedes." [link]
The full text of the Metropolitan Police report into the death of Diana is here - Operation Paget report.