The night was oppressively hot, and I lay in my bed at my weekend cottage in Dorset on the verge of sleep, when the silence was broken by the buzz of my pager at 4am. I fumbled around for it in the dark.
A message on the tiny screen demanded that I urgently contact Chief Superintendent Dai Davies of the Royalty And Diplomatic Protection Department.
There was no phone in my cottage, so I threw on some old clothes and walked to the telephone box a few minutes up the road. The sky was already lightening with the early dawn.
"I've got some bad news," the Chief Super said without preamble. What he told me next stunned me: the Princess of Wales was dead, killed in a road accident in Paris that night.
"I'd like you to return to London as soon as you can, to help co-ordinate the funeral arrangements," he added. Shocked, almost beyond speech, I said I was on my way, and replaced the receiver.
There have been times, since I left Diana's side in 1993, when I have questioned whether I was right to resign. This awful moment was the most poignant. The Princess, whom I had guarded for so many years, lay dead in a Paris hospital.
As I drove towards London, my mind kept returning to the same questions: Could anything have been done to save her? And how could this have happened? For the record, let me make clear that I had complete charge of the Princess's safety for nearly six years, while her bodyguard in Paris, Trevor Rees-Jones, was at her side for a matter of weeks.
Though terribly injured, he was the sole survivor of the crash that also killed Diana's lover, Dodi Fayed, and their driver, Henri Paul.
So, on behalf of all the professional men and women of the Met's protection squad, let me say that neither Rees-Jones nor any of the other bodyguards who attended Diana in the two months preceding her death were from our department.
I am still angry beyond words that this team of 'bodyguards' let her come to harm. Our department had the care of her personal safety for some 15 years: Fayed's crew were in charge of her security for just eight weeks before she died.
Rees-Jones was a former soldier who had not received the training necessary to protect a member of the Royal Family. When he first heard he'd been appointed by the Fayed family to guard Diana in France, he could have informally contacted Scotland Yard for a briefing.
Instead, according to his memoirs, he simply reflected that he was in for 'a hell of an interesting trip'.
Worryingly, he also bragged he was a "good bloke in a fight". That raises serious questions about his suitability. The ability to acquit oneself well in a brawl is not qualification enough to protect someone like Diana.
The primary role of a protection officer is to use intelligence, their contacts and their instincts to keep their charge out of harm's way by avoiding confrontation.
A case in point is Rees-Jones's lack of understanding of the paparazzi. He appeared to think in terms of his Army days, describing the Press as 'the enemy' and referring to photographers as if they were 'snipers' with their long lenses like rifle barrels.
He also seems to have been overawed by the Princess. In his book, he mentions how attractive he found her, and that he wanted to do things to please her. This is incompatible with a mission to protect. For the sake of Diana's safety, I could never be in awe of her.
Most seriously, Rees-Jones committed a grave error of judgment by allowing Dodi Fayed to enter into a game of 'beat the paparazzi', which led to the fatal high-speed crash in the Pont de l'Alma tunnel on August 31, 1997.
The origin of the tragedy, however, lay in Diana's rejection of the Queen's offer to keep her round-the-clock Scotland Yard protection in place. This inadvertently set off the chain of events that led to her own death.
If any Metropolitan Police protection officer had been with her, Diana would never have got into a car with a drunk driver: not only experience but common sense would not allow it.
The Princess, like most of the Royal Family, accepted her police protection officers as a fact of life - though she had little idea of the training required to do the job effectively.
With increasing irritation and incredulity, I listened over the years to the conspiracy theories promoted by Dodi's father, Mohamed Fayed, and his supporters.
I studied the official reports of the days and hours leading up to the crash. I can say with certainty, drawing on decades of police experience, that Diana's death was not murder but a dreadful accident that should have been avoided.
She was not the victim of shadowy figures who regarded her as an embarrassment to the Establishment, but of her boyfriend's erratic behaviour and her bodyguard's mistakes.
The first mistake was to use a bodyguard hired by the Fayed family, who was unable to say no to his employers. Dodi ordered Henri Paul to drive that night: the bodyguard should have stepped in, and refused to allow Diana into the car.
Dodi ordered the chauffeur to drive too fast: Rees-Jones should have countermanded that. A police protection officer wouldn't have hesitated to override Dodi's wishes.
I also question why the bodyguards put such emphasis on trying to shield the couple from paparazzi from the moment they arrived in Paris. It should have been far more important to focus on their physical safety. The paparazzi were firing flashguns, not bullets.
After landing at Le Bourget, Dodi took Diana to the villa in west Paris that was once home to the Duke of Windsor, the former Edward VIII, and now owned by Mohamed Fayed.
They arrived at 4pm and wandered the gardens, but then called on the security team to make arrangements to take them to the Ritz hotel, also owned by the Fayeds.
From a security point of view, staying in the villa rather than at the hotel would have made much better sense, and I would have urged them to consider this.
To add to the confusion, once at the Ritz, Dodi decided he wanted to return to his apartment on the Champs-Elysees to change for dinner. A scuffle with photographers took place outside as Diana left.
Emotions were running high on both sides, with the paparazzi eager to secure pictures and the couple anxious to be left in peace, but on Dodi's instructions they continued this unnecessary tour of Paris.
At this point, if I had been with Diana, I would have intervened. Rees-Jones and the other bodyguard, Kes Wingfield, didn't do that: they were paid employees of the man they were guarding, unable to ignore or gainsay his decisions.
That said, there was nothing to prevent them from telephoning the local gendarmerie to ask for back-up without informing Dodi first.
The Princess was now shuttling around Paris, in a highly charged emotional state. She was behaving irrationally and at times almost hysterically, her mood alternating between excitement and panic.
Later, they headed back to the Ritz for dinner, but were paranoid about photographers and, fearing a paparazzo would somehow see them, retired to their suite.
Then came the fatal error: Dodi ordered his team to take him and Diana to his Champs-Elysees apartment.
He ruled that his usual chauffeur, Philippe Dourneau, should drive the Range Rover away from the hotel as a decoy, while the couple were taken in a leased Mercedes driven by Henri Paul.
When Rees-Jones and Wingfield tried to protest, Dodi overruled them. The bodyguards lacked experience and authority.
So it was that the Princess was placed in a car with a driver who, the security team knew, had been drinking all day. Another of Fayed's bodyguards had noticed the smell of alcohol on him at lunch.
By 10pm, both Rees-Jones and Wingfield had seen Henri Paul downing pastis, a very strong spirit, in the Ritz bar - though apparently he told them it was pineapple juice. I am convinced that Scotland Yard protection officers would have detected immediately that he was drinking alcohol.
However, Paul was a senior man in the Fayed organisation, which may have inhibited the bodyguards.
As the Mercedes pulled away from the Ritz, Paul is said to have leaned out of the window to issue a 'catch us if you can' challenge to the waiting paparazzi. It was not the act of a sober driver.
It is clear that the bodyguard was alarmed at Paul's driving before the accident, for he tried to put on his seat belt. He failed to buckle it in time - what saved his life was the air bag.
But he should have instructed all the passengers - including the driver - to fasten their belts before they left the Ritz.
When I worked with the Princess, she would automatically fasten her belt the moment she got into the car, whether as driver or passenger. Yet neither she nor Dodi was wearing one at the time of the crash.
According to Rees-Jones, Henri Paul may have been so befuddled that he mistook the automatic gearbox for a manual. Trying to change gear, he threw the car into neutral, causing it to freewheel out of control and hit a pillar at more than 60mph.
In the depressing months that followed, conspiracy theorists led by Mohamed Fayed would claim that the accident was caused on purpose, by the driver of a Fiat Uno and a motorcyclist who blinded Paul with a flashgun.
This assassination was ordered, Fayed said, by the Royal Family and the British and U.S. intelligence agencies, supposedly to prevent Diana from marrying a Muslim.
I find that impossible to believe. In my professional opinion, there is no evidence to support the theory.
Furthermore, to stage a crash in a tunnel at night with a guarantee of killing all four people in the car would be impossible. Too many factors would have to be left to chance - factors that tragically combined, largely because of inadequate security, on the night.
In the aftermath, I was tasked with looking after security at Diana's funeral. The job was not easy, and Mohamed Fayed presented one of the first difficulties.
He kept trying to insist that, in keeping with his bizarre conspiracy theories, he was a target. He said he needed all his cumbersome, supposedly SAS-trained guards by his side in Westminster Abbey.
This was a ridiculous idea, as if he outranked the Queen or the Prime Minister, neither of whom had personal bodyguards beside them.
I took some pleasure in reminding his 'protection liaison official' that no heavy security presence would be permitted in the Abbey.
The funeral arrangements were fraught for me with security concerns and it was only as Elton John began to sing, filling the Abbey with his tribute in song to Diana, Candle In The Wind, that I had a moment to reflect.
The reverberations of her death travelled around the world, touching millions of lives, just as she had touched them when she was alive.
I hope people will remember her as she was at her best - a warm-hearted and fun-loving woman who really did make a difference.
It is probably a vain hope, but I owe it to her.