"A very small injury, but in the wrong place." Princess Diana's coroner on her unlikely death.

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Almost 22 years after the death of Princess Diana in a Paris tunnel, we now know exactly which injury was fatal.

Writing for the The Mail on Sunday, Dr Richard Shepard, a pathologist asked to review evidence of the 36-year-old’s death, says the former royal initially only appeared to suffer from a “few broken bones and a small chest injury”, but that a tiny tear in a vein in one of her lungs proved to be deadly.

Prince Harry on Princess Diana and Meghan Markle. Post continues after video. 

“To the ambulance services, she initially seemed injured but stable, particularly as she was able to communicate. While everyone focused on Rees-Jones, however, the vein was slowly bleeding into her chest,” he wrote.

“In the ambulance, she gradually lost consciousness. When she suffered a cardiac arrest, every effort was made to resuscitate her and in hospital she went into surgery, where they did identify the problem and attempted to repair the vein. But, sadly, by then it was too late,” he added.

The doctor explained that the brief period of consciousness was characteristic of a vital vein tear, as it is hidden away deep in the centre of the chest.

“Diana’s was a very small injury – but in the wrong place,” he wrote.

He believes that if she had been wearing a seat belt, she would have suffered from a black eye and a fractured arm, and that the crash would not have been fatal.

Princess Diana was in a car with her driver, Henri Paul, her bodyguard Trevor Rees-Jones, and her reported boyfriend, Dodi Fayed, speeding through a Paris tunnel as they were chased by paparazzi in August 1997.

It was the end of a summer of love, and a photo of Diana and Dodi kissing had sold for $2 million dollars, spurring a flurry of photographers to stalk the pair at any cost.

On 31 August 1997, Diana and her billionaire boyfriend dined at his father’s Ritz hotel in Paris. They were pursued by French paparazzi on motorbikes once they left the restaurant, prompting their crash into a pillar in the Pont de l’Alma tunnel.

Only her bodyguard, Trevor Rees-Jones, wore a seat belt and survived the crash.

Dodi was born in Egypt in 1955, the eldest son of Harrods tycoon Mohamed Al-Fayed and Samira Khashoggi. He divided his time between London, Paris, the Riviera and Cairo.

The pair met over a decade before they became romantically involved at a polo event that Diana attended with ex-husband Prince Charles in 1986. They also ran in the same circles in London.

Two months later after the couple became close on a luxury holiday, the fatal car accident occurred.