The UK's leading forensic pathologist, known for his work on Princess Diana's inquest, has disclosed that a single small action could have saved her life.

Dr. Richard Shepherd, renowned for conducting high-profile autopsies, has been involved in many significant investigations, such as the death of Stephen Lawrence, victims of 9/11 and 7/7, the Marchioness disaster, and the Hungerford massacre, among others.

The expert, whose new series Body In The Water debuted this week, has conducted over 23,000 post-mortems and has even overcome post-traumatic stress disorder, triggered by the tragic Bali Bombings in 2002.

However, the case Dr. Shepherd is most frequently questioned about is, understandably, the death of Princess Diana in August 1997.

For those who might not know, Diana, the Princess of Wales, was only 36 years old when she died following a car accident in Paris, France.

The royal was traveling through the Place de l'Alma underpass in central Paris in the rear of a Mercedes-Benz S280 with Egyptian film producer Dodi Fayed.

The forensic pathologist who worked on Princess Diana's inquest has revealed the one action that could have saved her (Tim Graham/Getty Images)

The pair were being chauffeured in the armored vehicle by Henri Paul, who was later found to be intoxicated by alcohol and under the influence of prescription drugs while driving. They were accompanied by Dodi’s bodyguard, Trevor Rees-Jones.

Initially, Diana was conscious and speaking after the crash, but she soon went into cardiac arrest. During surgery, medical staff identified the issue as a small tear in a crucial vein - a pulmonary vein.

Apart from this, she only sustained a few broken bones and a minor chest injury. Although the injury was small, it was in a critical location and was so rare that Dr. Shepherd had never encountered it before in his career.

As a top pathologist, Dr. Shepherd was asked to review the autopsy and provide evidence at the inquiry, an experience he described as 'like standing in the middle of a thunderstorm'.

Speaking to Tyla, he revealed that all it would have taken to save Princess Diana was just 'one click' - her seatbelt.

Except for Rees-Jones, the sole survivor, none of the passengers were wearing a seatbelt during the catastrophic crash.

Dr. Shepherd explained: "I was [surprised by Princess Diana’s death] because it was an extremely unusual injury, given that she was alive. She was talking. And when they reached the car, they noted she was the least injured person inside."

Dr. Richard Shepherd has worked on some of the most high-profile autopsies in the world (Tim Graham/Getty Images)

“The man in front of her, her bodyguard, Trevor Rees-Jones, was severely injured. He managed to fasten his seatbelt, but his face hit the dashboard, causing severe facial injuries and fractures."

Dr. Shepherd continued: "He was in quite a bad condition. Henri Paul and Dodi were both deceased in the seats next to her. They focused on Trevor Rees-Jones because Diana was trapped behind his seat, but she was talking and seemed okay.

"So it was one of those insidious injuries that eventually claimed her life."

He added: "It's so tragic that if only she had been wearing a seatbelt, she would have walked away. She might have had a broken rib or arm, but she would have survived the crash.

"Admittedly, it was a severe crash, but not excessively so, and Mercedes are designed to withstand such impacts. How tragic is it that the simple act of fastening a seatbelt could have saved her life?”

An official investigation by the Metropolitan Police concluded that, along with Paul being under the influence of alcohol, the vehicle was being pursued through Paris by paparazzi, eventually ruling the crash an unlawful killing.

Diana's death led to a global wave of grief and media attention, along with numerous conspiracy theories, the most prominent suggesting royal family involvement.

However, Dr. Shepherd's response to such theories is straightforward - they are incorrect.

"It was so tragic that if only she had worn a seatbelt, she would have walked out" (Bettmann/Getty Images)

"Well, I'm afraid I would just shrug my shoulders and say, well, you're wrong," he stated, "It's always possible to take 20 facts and weave them into some kind of narrative."

He mentioned people asking for explanations of details, like 'Why was it the 13th pillar in the tunnel?'

The pathologist's reply is simple: "Well, that's random, you know, it is just one of those things. If you were part of a 'Prince Philip Assassination Squad', you wouldn't deliberately crash them into the 13th pillar. You’d do something else. So it just doesn't hold water in the context of what was happening.

"Henri Paul was drunk, he wasn't a skilled driver, and he was intent on outrunning the paparazzi, possibly with Dodi in the back urging, 'Just go faster, just go faster'.

"You can imagine the scenario unfolding, and a bad driver, partially drunk in a car at that speed, is not going to end well."

Body In the Water airs from 16th September on Tuesdays at 10 pm on the TRUE CRIME channel, with episodes available to stream on WATCH FREE UK following linear broadcast, and via TRUE CRIME UK on ITVX.

Dr Richard Shepherd is also about to visit theatres up and down the UK with his new show - Dr Richard Shepherd -Time of Death - More Unnatural Causes. You can get tickets here.