Inevitably, the state-controlled media in Saddam Hussein's Iraq and Muammar Gaddafi's Libya were quick to jump on the bandwagon, saying she had been assassinated by the security services.
But Mohamed Al-Fayed, the father of Dodi Fayed and former owner of Harrod's, has repeatedly claimed that his son and Diana were murdered. In 2001 he claimed that Diana was pregnant with Dodi's son.
An 18-month French judicial investigation concluded in 1999 that the crash was caused by Fayed's deputy head of security Henri Paul, who lost control at high speed while intoxicated.
He was found to have had three times the legal limit for alcohol in his system, and was driving at more than double the speed limit.
One colleague of 41-year-old Paul said the security man was "drunk as a pig" as he got into the car. Another claimed he had taunted the paparazzi "You won't catch me."
Within hours of the crash, the Iraqi newspaper Babel reported: "Diana was liquidated by the British secret service because she departed from the norms and dabbled in politics, although she is from a family which is supposed to reign but not govern.
The paper, run by Saddam Hussein's son Uday, said Diana and Dodi Fayed were being driven at break-neck speed not to escape photographers, but because they were being 'terrorised by assassins'.
"The liquidation took place in France for the British secret service to escape responsibility," the paper said.
Gaddafi said the crash was arranged by the British and French security services to make sure a member of the British Royal Family did not marry an Arab.
"If Diana had given birth to a child of al-Fayed, he would have been the brother of her sons from Prince Charles," said Gaddafi.
"It is very clear that they did not want the brother of a British prince to be an Arab Muslim. Therefore, they had to kill al-Fayed and Diana."
French Judge Hervé Stephan, who oversaw the official investigation into the crash, cleared the paparazzi photographers of responsibility for the crash. He concluded they were some distance from the car when it crashed and could not be blamed.
In 2008 a British inquest jury returned a verdict of "unlawful killing" by driver Henri Paul and the paparazzi pursuing the car.
The jury's verdict also stated: "In addition, the death of the deceased was caused or contributed to by the fact that the deceased were not wearing a seat belt and by the fact that the Mercedes struck the pillar in the Alma Tunnel rather than colliding with something else".
A special Metropolitan Police inquiry team was established in 2004, Operation Paget, headed by then-commissioner John Stevens to investigate the various conspiracy theories which led up to the British inquest. This investigation looked into 175 conspiracy claims that had been made by Fayed, but all were rejected.
One of the claims surrounded the white Fiat Uno which clipped the Mercedes Diana and Dodi were travelling in moments before the crash.
In 2005, Al-Fayed said the Fiat was being driven by an MI6 officer, and later elaborated that it belonged to French photographer, Jean-Paul James Adanson, who had killed himself in 2000. Al-Fayed said Adanson either took his own life due to guilt over his part in the crash, or was assassinated by the French or British security services to silence him. Operation Paget said that Adanson's Fiat was in an unroadworthy condition at the time of the crash, and the fact that he had openly sold it a few weeks later suggested it was highly unlikely to have been the car the Mercedes collided with.
Another theory is that the Fiat was driven by Le Van Tanh, a 22-year-old taxi driver at the time of the crash. In 2006, Thanh's father said his son had re-sprayed his white Uno red hours after the crash, allegedly waking up his mechanic brother in the night to help him. Following tests, it was concluded that the car "could have been involved in the accident", but Thanh's involvement was ruled out by French police because he said he was at work on the night in question. It was later alleged that he had left work early that night and could have been at the scene of the crash. Multiple witnesses recall seeing a man matching his description exit the tunnel seconds after the crash.
Another theory involves a flash of light which blinded Paul as he drove towards the tunnel.