The black Mercedes-Benz S-Class carrying the Princess, her companion Dodi Fayed, bodyguard Trevor Rees-Jones and driver Henri Paul crashed into the 13th pillar in the tunnel de l'Alma at approximately 12:23am on August 31, 1997.
The car was being chased through the streets of Paris by paparazzi on motorcycles, nine of which were charged with manslaughter but later released.
Dr Frederic Mailliez was on his way home from a birthday party, when he noticed the smoking wreckage.
He told GB News: "Inside the car, there were four people. The passenger in the front right was screaming [and] alive."
Dr Frederic Mailliez talking to Cameron Walker GB News
Princess Diana John Stillwell
Bodyguard Trevor Rees-Jones was the only survivor of the crash and Dr Mailliez, the only medical professional at the scene, made the split-second decision to prioritise another victim who was less likely to live.
Dr Mailliez told GB News Henri Paul, the driver, appeared dead and was underneath the steering wheel.
Diana's companion, Egyptian film producer Dodi Fayed, was also lifeless.
The off-duty doctor didn't recognise Diana.
He said the "young lady was on her knees on the floor of the car, and she was almost unconscious with difficulty breathing".
Dr Mailliez continued: "So I did a very quick medical assessment. I called the emergency services and I ran back to my car."
Dr Mailliez grabbed the basic medical kit he had in the boot and climbed into the wreckage next to the princess.
He said: "I sat down on the rear seat and I cautiously lifted up [Princess Diana's] head and applied the respiratory bag to help her breathe.
"I understand that people want to know what her condition [was like], but I always refuse to describe in detail the injuries because I always thought about her family, especially her two sons.
"I [didn't want to] add more pain."
After being told that the victims spoke English, Dr Mailliez told Diana that he had called an ambulance and everything would be okay.
He told GB News: "I'm probably the last person that she heard."
Sitting in the car, next to the dying mother of a future King, Dr Mailliez noticed the flash of paparazzi camera bulbs taking pictures.
He said: "As the only doctor in front of four [severely injured] victims,I didn't have time to [wonder] why there were so many photographers."
Diana was rushed to La Pitié-Salpêtrière Hospital where she underwent emergency surgery.
The 36-year-old was pronounced dead at 4am.
In the morning, Dr Mailliez saw the news and realised that the woman he had treated hours earlier in a Paris underpass was Diana, Princess of Wales.
He said: "That was a big shock, because I knew that in this kind of very high energy impact accident you can expect some severe lesions, severe injuries, but I didn't think she would die so quickly."
A jury in the 2008 inquest into the deaths of Princess Diana and Dodi Fayed found that they had been unlawfully killed.
The "speed and manor" of Henri Paul's driving whilst under the influence of alcohol, photographers following on motorbikes, and the fact no seatbelts were worn were all found to be contributing factors.
Cameron Walker in conversation with Dr Frederic Mailliez GB News
Cameron visited Paris on the anniversary of Princess Diana's death GB News