'Hugs can do a great amount of good – especially for children.'

'THIS IS THE THIRTY-SECOND WARNING.' Giving Diana's police T security code I spoke quietly and without inflexion into the radio, then returned it to the side of my seat in the Jaguar. We never signalled the arrival of the principal any earlier, as to have done so would have been poor security. Radio calls are easily monitored.

'Oh Ken, anyone would think the world was about to end.' The woman in the back of the car always found our procedure amusing. 'For goodness' sake, it's only little old me coming home,' she added with a girlish tickle in her voice.

Seconds later the dark green Jaguar XJ6 approached the police security barrier and, after being waved through by the constable on duty, swept into Kensington Palace. In the back of the car sat the most famous woman in the world, Diana, Princess of Wales. I always sat in the front, next to her trusted chauffeur – and my friend – Simon Solari. We were the only people in the car.

Only half an hour earlier, the Princess and I had stepped from Concorde at Heathrow after a flight from Dulles International airport, Washington, DC. Exhilarated, she had hardly been able to sit still in her designated seat, and had talked non-stop about her charity mission to America, during which she had made a friend of the First Lady, Barbara Bush. Indeed, the President himself had delayed a meeting to chat with Diana, and his decision to join them had impressed her. On that trip we had travelled under the names Mr and Mrs Hargreaves, although none of Concorde's crew had been remotely fooled. At the time she was already beginning to revel in her success as a royal personage – in fact, an international celebrity – in her own right.

Now we were back at Numbers 8 and 9 Kensington Palace, her official London residence. The Prince of Wales, her husband, was not waiting for her inside.

'Home sweet home,' Diana sighed, with more than a hint of irony, although without bitterness.

It was just after 10.35 pm on a dark autumn night in 1991, and I had been doing this job for nearly three years. The Princess, however, had been doing hers for nearly ten.

There are not many days when I do not think of Diana, Princess of Wales. Her illuminating smile, her sheer presence, and above all her yearning to live life to the full, have never left me. I am sure I am not the only person to be haunted in this way, although, with each year that passes since her death, fashionable opinion seems increasingly to insist that our memory of the once vivid woman has become not only more distant, but more uncertain. There is no shortage of commentators who want us to believe that the Diana legend is fading, or even that the substance behind that legend was of little worth.

The Diana I knew was full of fun, almost always in search of laughter, not wallowing in self-pity and tears as she is now so often portrayed. There were, of course, dark clouds in her life, but they would soon pass to allow her nature to shine brightly once again. Yet since her death on 31 August 1997 history has all too often presented a very different – not to say distorted – image of this extraordinary woman. Worse, since 1997, a PR offensive has been waged in some quarters against a dead woman's memory. Her name has been dragged through the mud, her principles derided, her motives corrupted, and even her sanity questioned. It has been, in my view at least, a vicious and one-sided war, and as in any war, the truth has been the first casualty.

For nearly five years, from 1988 to 1993,1 shadowed the late Princess in my capacity as her Scotland Yard personal protection officer (PPO, or, in layman's terms, her police bodyguard), during the most traumatic period of her life. For most of that time she was a joy to work with. As her senior protection officer it fell to me to deal with her more sensitive private engagements and public appearances, and my relationship with her was, by the very nature of the job, an extremely close one. Due to the unique position in which I found myself, however, it was inevitable that my duties could not always be clearly defined. Naturally she and I freely discussed all matters affecting her security, but we also talked openly about her life, including the most intimate aspects of it. Consequently, during my time with her, I was not only her police officer but also a trusted aide and confidant.

While this may seem a conceited view, it has its roots in the nature of my profession: if I had not liked and trusted her, I could not have done my job efficiently, while she would hardly have tolerated a protection officer whom she did not trust. There was an open-door policy between us; my independence, given that I worked not directly for her but for Scotland Yard, meant I could and would always speak freely and truthfully, unlike the army of courtiers employed by her husband, the Prince of Wales, and by the Queen. The uniqueness of my role gave me, I believe, an unrivalled appreciation of the true Diana, the woman behind the public mask.

I am the first officer from Scotland Yard's elite Royalty and Diplomatic Protection Department to publish an insider's account of service with that department. This may seem an empty claim, but I believe it is a unique story because it was shared with Diana. Most of our experiences were known only to the two of us. I would never have put pen to paper had the Princess been alive today. Since her death, however, I have become increasingly concerned at the way she has been portrayed by the media, by journalists and writers, even those who claim to have known her well. I have come to feel that unless I tell of my years with her as I saw them through my policeman's eyes, then people now and in the future will receive a corrupt impression of her.

My intention in writing this book is simple – to set the record straight about the woman who herself once claimed that I knew her better than anyone, and in doing so, to tell the simple truth about one of the most remarkable, complex and alluring public figures of the latter part of the twentieth century. When I joined the Metropolitan Police service as a special cadet in 1964 at the tender age of sixteen, I never dreamed that I would one day shadow one of the world's most famous women. A chance meeting with an old friend sent me on a journey that changed my life for ever.

At the age of thirty-four I was already a police inspector, more or less assured of a rapid rise through the hierarchy at Scotland Yard. One evening in the summer of 1986 I arranged to have a quiet drink with Jim Beaton, a clever and experienced officer, who had been honoured with the George Cross for saving Princess Anne, and who had been a chief inspector at Kensington police station when I was a 'skipper' (sergeant) there in the late 1960s. I had always trusted him as a colleague, and, I suppose, regarded him as my mentor. He had had a varied and successful career since our days at Kensington together, and had been promoted to the rank of superintendent. He was also the Queen's personal protection officer. As we sipped glasses of his favoured single-malt whisky, Jim came to the point. In confidence, he told me that his department, Royalty Protection, was looking for a police inspector to take charge of guarding the Queen's grandchildren, Princes William and Harry, the sons of the Prince and...