Princess Diana would have had an 80 percent chance of surviving the car crash that took her life if she had worn a seat belt, according to a policeman who took part in the official investigation into the tragedy.

David Douglas, a former senior officer in the Metropolitan Police, helped put together the official Operation Paget report into the circumstances surrounding the 1997 crash in the Pont de l'Alma tunnel in Paris.

Diana and her boyfriend Dodi Fayed were both killed along with the driver, Henri Paul, acting head of security at the Ritz hotel. Douglas spoke out ahead of the 25th anniversary of their deaths.

The intervention comes against the backdrop of longstanding but debunked conspiracy theories suggesting Diana's death was a deliberate assassination.

Douglas told Good Morning Britain on August 19: "When you look at most incidents [and] accidents, you find there's a chain of events. And if any one of those chain of events had been different it might not have led to that happening…

"For example, if they had been wearing seat belts, our experts tell us there was probably an 80 percent chance that they would have survived the accident. It would still have been a terrible accident, they would have been badly injured, but it probably would not have been fatal."

He added that the presence of alcohol in the driver's blood may have contributed to the crash, along with the presence of paparazzi photographers following the car.

Douglas said: "Yes, Henri Paul had been drinking alcohol. He certainly wasn't drunk, we've never said that. But we all know that if you have any alcohol it impairs you ability to drive."

Princess Diana's death was initially investigated by French police but London's Metropolitan Police launched its own investigation on the request of the coroner.

The move followed a letter from Mohamed Al Fayed, Dodi's father, alleging the deaths were a murder carried out by the security services on the orders of Prince Philip.

The official Operation Paget report stated: "Mohamed Al Fayed has made a principal crime allegation, supplemented by numerous linked claims and assertions.

"In essence Mohamed Al Fayed's allegation is that the 'Security Services' (unless otherwise specified, this is taken to be the Secret Intelligence Service (SIS)—commonly known as MI6) acting at the behest of HRH Prince Philip, arranged for or carried out the murder of Dodi Al Fayed and the Princess of Wales.

"The alleged motive was that the Princess of Wales was pregnant with Dodi Al Fayed's child and there was to be an imminent announcement of their engagement.

"It is suggested by Mohamed Al Fayed that the Royal Family 'could not accept that an Egyptian Muslim could eventually be the stepfather of the future King of England.'"

Theories that the crash was an assassination centered around speculation the driver may have been blinded by a bright light shone from a motorcycle inside the dark Pont de l'Alma tunnel.

However, Operation Paget debunked this idea by citing physical evidence, such as tire marks on the road, which demonstrated driver Henri Paul began to lose control 30 to 60 meters before the car entered the tunnel.

The report stated: "It is apparent that there was a loss of control of the Mercedes some distance before the Alma underpass.

"The loss of control had already commenced before that location and therefore any bright lights or flashes on the immediate approach to or within the underpass were not a contributory factor to that loss of control.

"This means there is no significance in any flashes in the underpass. This is an important point in relation to the conspiracy allegation."

The official inquest found the deaths were an unlawful killing as a result of gross negligent driving contributed to by paparazzi who were following the car.

The forewoman of the jury told the Royal Courts of Justice in 2008: "The crash was caused or contributed to by the speed and manner of the driving of the Mercedes, the speed and manner of driving of the following vehicles, the impairment of the judgment of the driver of the Mercedes through alcohol, and there are nine of us who agree on those conclusions.

"In addition, the death of the deceased was caused or contributed to by the fact that the deceased [were] not wearing seat belt(s), the fact that the Mercedes struck the pillar in the Alma Tunnel rather than colliding with something else—and we are unanimous on that, sir."

Join half a million readers enjoying Newsweek's free newsletters