Devastated, she decided to make her side of the story public by recording her thoughts for author Andrew Morton via a go-between. Her one condition: that her involvement be kept a strict secret. The book he wrote - Diana: Her True Story - caused a sensation, the Daily Mail reports.
Now, 20 years after her death, it is being republished, with transcripts of those tapes. Our first extract begins with her meeting Charles at her home, Althorp House in Northamptonshire, in November 1977, when she was just 16. He was 29 - and at the time dating her sister Sarah, 22 . . .
Charles came to Althorp to stay, and the first impact was: "God, what a sad man." He came with his labrador.
My sister Sarah was all over him like a bad rash, and I thought: "God, he must really hate that.'
I kept out of the way. I remember being a fat, podgy, no make-up, unsmart lady but I made a lot of noise, and he liked that. And he came up to me after dinner and we had a big dance, and he said: "Will you show me the gallery?" (The 115ft-long Picture Gallery contained Van Dyck's War And Peace, among other paintings).
And I was just about to show him the gallery and Sarah comes up and tells me to push off. I said: "At least let me tell you where the switches are to the gallery, because you won't know where they are," and I disappeared.
And he was charm himself. And when I stood next to him the next day, a 16-year-old, for someone like that to show you any attention - I was just so sort of amazed. "Why would anyone like him be interested in me?" And it was interest.
That was it for about two years. I saw him off and on with Sarah (whose relationship with Charles ended after nine months). When he had his 30th birthday dance (at which The Three Degrees performed, in November 1978 at Buckingham Palace) I was asked, too.
"Why is Diana coming as well?" my sister asked. I said: "Well, I don't know, but I'd like to come." I had a very nice time at the dance - fascinating. I wasn't at all intimidated by the surroundings. I thought: amazing place. Then I was asked to stay at the de Passes (friends of Prince Philip) in July 1980 by Philip de Pass, who is the son.
"Would you like to come and stay for a couple of nights down at Petworth (in West Sussex), because we've got the Prince of Wales staying. You're a young blood - you might amuse him."
So I said: "OK."
Charles came in. He was all over me again, and it was very strange.
I thought: "Well, this isn't very cool." I thought men were supposed not to be so obvious; I thought this was very odd.
The first night, we sat down on a hay bale at the barbecue at this house, and he'd just finished with Anna Wallace (the daughter of a Scottish landowner, known - on account of her fiery temper - as Whiplash Wallace, with whom he had a stormy, six-month relationship).
I said: "You looked so sad when you walked up the aisle at Lord Mountbatten's funeral (Charles's beloved great-uncle had been killed in County Sligo, Ireland, the year before, in August 1979, by an IRA bomb on his boat).
"It was the most tragic thing I've ever seen. My heart bled for you when I watched. I thought: 'It's wrong, you're lonely - you should be with somebody to look after you.' "
The next minute, he leapt on me practically, and I thought this was very strange, too, and I wasn't quite sure how to cope with all this. Frigid wasn't the word. Big F, when it comes to that.
We talked about lots of things, and anyway that was it. He said: "You must come to London with me tomorrow. I've got to work at Buckingham Palace - you must come to work with me." I thought this was too much. I said: "No, I can't."
I thought: "How will I explain my presence at Buckingham Palace when I'm supposed to be staying with Philip (de Pass)?" Then (two weeks later, in early August) he asked me to Cowes on Britannia (the royal yacht), and he had lots of older friends there and I was very intimidated.
But they were all over me like a bad rash. I felt very strange about the whole thing.
Then I went to stay with my sister Jane at Balmoral (for the weekend of the Braemar Games in early September) where Robert (Fellowes, sister Jane's husband) was assistant private secretary (to the Queen).
I was terrified, because I had never stayed at Balmoral and I wanted to get it right. The anticipation was worse than actually being there. You're all right once you get in through the front door. I had a normal single bed! I'm just telling you.
I have always done my own packing and unpacking. Now, obviously, I don't - I haven't got the time. But I was always appalled that Charles takes 22 pieces of hand luggage with him. That's before all the other stuff. I always have four or five. I felt rather embarrassed.
Mr and Mrs Parker Bowles were there at all my visits. I was the youngest there by a long way. Charles used to ring me up and say: "Would you like to come for a walk, come for a barbecue?"
So I said: "Yes, please."
I thought this was all wonderful.
Chief chick - and a girl about town
When Diana turned 18, her parents bought her a three-bedroom flat in South Kensington as a lavish coming-of-age present. She shared it with three girlfriends - Virginia Pitman, Carolyn Bartholomew and Ann Bolton. On her bedroom door were emblazoned the words Chief Chick. She also worked as a nanny and in a kindergarten.
It was nice being in a flat with the girls. I loved that - it was great. I laughed my head off there. I kept myself to myself. I wasn't interested in having a full diary. I loved being on my own, as I do now - a great treat.
They (her nannying jobs) were often pretty grim employers - velvet hairbands. I was sent out to all sorts of people from my sisters - their friends were producing rapidly. They sent me out the whole time - it was bliss.
Solve Your Problems (employment agency) sent me on cleaning missions, but nobody ever thanked me for it. But that was just a fill-in on Tuesdays and Thursdays, because Mondays, Wednesdays and Fridays I worked in a kindergarten. So I had two jobs, which was great.
I did a cookery course in Wimbledon with Mrs Russell. She's French. I quite liked it, but more velvet hairbands. I got terribly fat because my fingers were always in the saucepans, for which I got fined.
It wasn't my idea of fun, but my parents wanted me to do it.
At the time it seemed a better alternative than being behind a typewriter - and I got a diploma!
The country house... near Camilla's home
Charles said he wanted to be in the Duchy of Cornwall vicinity, but it's only 11 miles from Camilla's house. He chose the house and I came along afterwards.
I first went there after he bought it. He had painted all the walls white. He wanted me to do it up, even though we were not engaged. I thought it was very improper, but he liked my taste.
(Interior designer) Dudley Poplak did up my mother's house ten years previously and had always been a friend of my mother's, so I said to her: "What do you think?" She said: "Well, use him - he's been marvellous, very loyal."
I chose the decorations and had a free hand to do that.
Giggline as the Prince proposes
On February 3, 1981, Prince Charles, then 32, proposed to Diana in the nursery at Windsor Castle. She was 19. Their engagement was announced three weeks later, on February 24. That same day, in a BBC interview, Charles, when asked if they were in love, famously responded: "Whatever in love means."
Before nightfall, Diana had moved out of Coleherne Court, her South Kensington flat, and into the Queen Mother's residence, Clarence House, on The Mall. That wasn't the only radical change in her life. In the six months leading to the engagement, she - and her flatmates - had come increasingly under Press scrutiny.
Our relationship sort of built up from there (the Braemar Games weekend). Then the Press seized upon it.
Then that became simply unbearable in our flat, but my three girls were wonderful - star performers, loyalty beyond belief. The feeling (in Sandringham) was: I wish Prince Charles would hurry up and get on with it. The Queen was fed up.
He wrote to me from Klosters (the Swiss skiing resort) and then he rang me up and said: "I've got something very important to ask you."
An instinct in a female tells you what it is. I sat up all night with my girls, saying: "Christ, what am I going to do?"
Diana had regularly asked her flatmates for advice on how to conduct her romance. Carolyn Bartholomew recalled: "It was pretty normal procedure that goes on between girls. Some of it I can't disclose, some of it would have been on the lines of: 'Make sure you do this or that.' It was a bit of a game."
By that time, I'd realised there was somebody else around. I'd been staying at Bolehyde (Manor, home of Camilla and her husband Andrew) with the Parker Bowleses an awful lot, and I couldn't understand why she kept saying to me: "Don't push him into doing this, don't do that."
She knew so much about what he was doing privately and about what we were doing privately . . . like if we were going to stay at Broadlands (the Mountbatten family seat in Hampshire).
I couldn't understand it. Eventually, I worked it all out and found the proof of the pudding, and people were willing to talk to me.
Anyway, next day I went to Windsor and I arrived about 5 o'clock and he sat me down and said: "I've missed you so much."
But there was never anything tactile about him. It was extraordinary, but I didn't have anything to go by because I had never had a boyfriend.
I'd always kept them away, thought they were all trouble - and I couldn't handle it emotionally. I was very screwed up, I thought.
Anyway, so he said: "Will you marry me?" and I laughed. I remember thinking: "This is a joke", and I said: "Yeah, OK", and laughed.
He was deadly serious. He said: "You do realise that one day you will be Queen?"
And a voice said to me inside: "You won't be Queen, but you'll have a tough role."
I thought: "OK", so I said: "Yes." I said: "I love you so much, I love you so much."
He said: "Whatever love means"
He said it then. I thought that was great! I thought he meant that! And so he ran upstairs and rang his mother.
In my immaturity, which was enormous, I thought that he was very much in love with me, which he was. He always had a sort of besotted look about him, looking back at it, but it wasn't the genuine sort. Who was this girl who was so different? But he couldn't understand (me) because his immaturity was quite big in that department, too.
(later) A briefcase comes along on the pretext that (Prince) Andrew is getting a signet ring for his 21st birthday and along come these sapphires. I mean nuggets! I suppose I chose it, we all chipped in. The Queen paid for it.
Flatmates screaming about her big news
It was only after they were engaged that Diana was allowed to call Prince Charles by his first name; until then she had called him Sir. This was considered normal by his circle at the time - indeed, her elder sister Sarah had also called him Sir when she'd gone out with him.
For me, it was like a call of duty, really - to go and work with the people.
I came back to the flat and sat on my bed. "Girls, guess what?"
They said: "He asked you. What did you say?" "Yes, please." They screamed and howled and we went for a drive around London with our secret.
I rang my parents the next morning. Daddy was thrilled: "How wonderful." And Mummy was thrilled.
I then went away two days later to Australia for three weeks to sort of settle down and to organise lists and things with my mother. That was a complete disaster because I pined for him, but he never rang me up.
I thought that was very strange, and whenever I rang him, he was out and he never rang me back. I thought: "OK" - I was just being generous - "He is being very busy, this, that and the other."
I come back from Australia, someone knocks on my door - someone from his office with a bunch of flowers, and I knew that they hadn't come from Charles because there was no note. It was just somebody being very tactful in the office.
Then it all started to build up, sort of like the Press were being unbearable, following my every move.
I understood they had a job, but people did not understand they had binoculars on me the whole time. They hired the opposite flat in Old Brompton Road, which looked into my bedroom, and it wasn't fair on the girls.
I had to get out once to go to stay with Charles at Broadlands. So we took my sheets off the bed and I got out of the kitchen window, which is on the side street, with a suitcase. I did it that way round.
I was constantly polite, constantly civil. I was never rude. I never shouted. But I cried like a baby to the four walls. I just couldn't cope with it. I cried because I got no support from Charles and no support from the Palace press office. They just said: "You're on your own", so I thought: "Fine."
Charles wasn't at all supportive. Whenever he rang me up, he said: "Poor Camilla Parker Bowles. I've had her on the telephone tonight and she says there's lots of press at Bolehyde. She's having a very rough time."
I asked him: "How many press are out there?" He said: "At least four."
I thought: "My God, there's 34 here!' and I never told him. I never complained about the Press to him because I didn't think it was my position to do so.
I was able to recognise an inner determination to survive. Anyway, thank God, the engagement got announced and before I knew what happened, I was in Clarence House.
Nobody there to welcome me. It was like going into a hotel. Everyone said: "Why are you at Clarence House?" and I said I was told that I was expected to be at Clarence House. I remember being woken in the morning by a very sweet elderly lady who brought in all the papers about the engagement and put them on my bed.
I'd left my flat for the last time and suddenly I had a policeman. And my policeman the night before the engagement said to me: "I just want you to know that this is your last night of freedom ever, in the rest of your life, so make the most of it."
It was like a sword went in my heart. I thought: "God", then I sort of giggled like an immature girl.
A tricky lunch with Camilla
I met her very early on. I was introduced to the circle - but I was a threat. I was a very young girl, but I was a threat.
When I arrived at Clarence House, there was a letter on my bed from Camilla, dated two days previously, saying: "Such exciting news about the engagement. Do let's have lunch soon when the Prince of Wales goes to Australia and New Zealand. He's going to be away for three weeks. I'd love to see the ring. Lots of love, Camilla."
And that was: "Wow!" So I organised lunch. Bearing in mind that I was so immature, I didn't know about jealousy or depressions or anything like that.
I had such a wonderful existence being a kindergarten teacher - you didn't suffer from anything like that. You got tired, but that was it. There was no one around to give you grief.
So we had lunch. Very tricky indeed. She said: "You are not going to hunt, are you?"
I said: "On what?"
She said: "Horse. You are not going to hunt when you go and live at Highgrove, are you?" I said: "No."
She said: "I just wanted to know."
And I thought as far as she was concerned, that was her communication route. Still too immature to understand all the messages coming my way.
Isolated at Buckingham Palace
After a few days, Diana moved from Clarence House to Buckingham Palace. It was a place of "dead energy", she said, and she felt lonely there. She regularly wandered from her second-floor apartment to the kitchens to chat to the staff - and on one occasion, barefoot and in jeans, buttered toast for an astonished footman.
I couldn't believe how cold everyone was; how I thought one thing but actually another thing was going on.
The lies and the deceit! The first thing that hit me was my (future) husband sending Camilla Parker Bowles flowers when she had meningitis: "To Gladys from Fred": (their nicknames for each other).
I never dealt with that side of things. I just said to him: "You must always be honest with me."
I was the only one here (when planning the wedding) because (Charles) had pushed off to Australia and New Zealand on tour, and you may recall, of course, the picture of me sobbing in a red coat when he went off in the aeroplane.
It had nothing to do with him going. The most awful thing had happened before he went. I was in his study talking to him, when the telephone rang. It was Camilla.
I thought: "Shall I be nice (and leave him alone so he can talk to her in private) or shall I just sit here?" So I thought I'd be nice, so I left them to it. It just broke my heart, that.
We always had discussions about Camilla, though. I once heard him on the telephone in his bath on his hand-held set, saying: "Whatever happens, I will always love you."
I told him afterwards that I had listened at the door, and we had a filthy row.
Somebody in his office told me that my husband had had a bracelet made for her, which she wears to this day. It's a gold chain bracelet with a blue enamel disc. It's got "G and F" entwined in it, "Gladys" and "Fred".
I walked into this man's office one day and said: "Oh, what's in that parcel?"
He said: "Oh, you shouldn't look at that."
I said: "Well, I'm going to look at it."
I opened it, and there was (the) bracelet, and I said: "I know where this is going." I was devastated. This was about two weeks before we got married.
He said: "Well, he's going to give it to her tonight."
So rage, rage, rage! "Why can't you be honest with me?" But, no, Charles cut me absolutely dead. It's as if he had made his decision; and if it wasn't going to work, it wasn't going to work.
He'd found the virgin, the sacrificial lamb, and in a way he was obsessed with me. But it was hot and cold, hot and cold. You never knew what mood it was going to be - up and down, up and down.
He took the bracelet, lunchtime on Monday. We got married on the Wednesday. I went to his policeman, who was back in the office, and said: "John, where's Prince Charles?" and he said: "Oh, he's gone out for lunch."
So I said: "Why are you here? Shouldn't you be with him?"
"Oh, I'm going to collect him later."
So I went upstairs, had lunch with my sisters who were there, and said: "I can't marry him. I can't do this. This is absolutely unbelievable."
They were wonderful and said: "Well, bad luck, Duch (her childhood nickname), your face is on the tea-towels so you're too late to chicken out."
The bulimia started the week after we got engaged (and would take nearly a decade to overcome).
My husband put his hand on my waistline and said: "Oh, a bit chubby here, aren't we?" and that triggered off something in me. And the Camilla thing. I was desperate, desperate. I remember the first time I made myself sick. I was so thrilled because I thought this was the release of tension.
The first time I was measured for my wedding dress, I was 29 inches around the waist. The day I got married, I was 23½ inches. I had shrunk into nothing from February to July. I had shrunk to nothing.
Dealing with Charles' chums
They appeared at various events, like the opera and going to Annabel's (nightclub) afterwards.
The circuit then was Jeremy and Sue Phipps, Charlie and Patti Palmer-Tomkinson, Camilla and Andrew Parker Bowles, Emilie and Hugh van Cutsem, Simon and Annabel Elliot - Camilla's sister and brother-in-law. They were the big speakers. Then there were some on the outside, too.
I started to think: "Gosh, they talk rather strangely to me."
I was very normal in the sense that I said what I thought, because nobody ever told me to shut up. They were all oiling up, basically, kissing Charles's feet, and I thought it was so bad for an individual to receive all that. Emilie van Cutsem used to be my best friend. She told me about Camilla. She's very formidable, very outspoken. Now the circle has broadened.
Other people have come in and they're not so much of a threat. They're actually terribly nice to me. I get on very well with them. But the ones who were there at the beginning are the ones that rumble a lot.
A disastroud first royal night out
In March 1981, Diana accompanied Prince Charles on their first joint public engagement, a black-tie event at Goldsmith's Hall in aid of the Royal Opera House development appeal. She wore a strapless, black taffeta evening gown designed by Elizabeth and David Emanuel, who would go on to design her wedding dress.
I remember my first engagement so well. So excited. I got this black dress from the Emanuels and I thought it was OK because girls my age wore this dress. I hadn't appreciated that I was now seen as a royal lady, although I'd only got a ring on my finger as opposed to two rings. I remember walking into my husband-to-be's study, and him saying: "You're not going in that dress, are you?"
I replied: "Yes, I am."
And he said: "It's black! But only people in mourning wear black!" And I said: "Yes, but I'm not part of your family yet."
Black, to me, was the smartest colour you could possibly have at the age of 19. It was a real grown-up dress. I was quite big-chested then, and they (the Press) all got frightfully excited.
I learned a lesson that night. I remember meeting Princess Grace and how wonderful and serene she was - but there was troubled water under her, I saw that.
(Princess Grace noticed how frightened she was and whisked her off to the ladies' for a chat. Diana poured her heart out to her about her sense of isolation and her fears for the future. "Don't worry," Princess Grace joked. "It will only get worse.")
It was a horrendous occasion. I didn't know whether to go out of the door first. I didn't know whether your handbag should be in your left hand or your right hand.
I was terrified, really - at the time everything was all over the place. I remember that evening so well. I was terrified - nearly sick.
I missed my girls so much. I wanted to go back there (to Coleherne Court) and sit and giggle like we used to, and borrow clothes and chat about silly things - just being in my safe shell again.
One day, you've got the King and Queen of Sweden coming to give you their wedding present of four brass candlesticks, the next minute you get the President of Somewhere Else coming.
I was just pushed into the fire. But I have to say, my upbringing was able to handle that. It wasn't as though I was picked out - like My Fair Lady - and told to get on with it. I did know how to react.
You see, I had a very good lifestyle myself. I was Lady Diana Spencer. I was living in a big house, I had my own money. So it wasn't as though I was going into anything different.