Prince Charles told Diana he "refused to be the only Prince of Wales who never had a mistress" when she confronted him over his affair with Camilla, an explosive documentary has revealed.
The Prince and Princess of Wales' marriage turmoil and sex life is laid bare in a new Channel 4 programme, drawing on taped sessions between Diana and her voice coach.
Diana pours out her heart during the controversial interviews and reveals that she begged the Queen for marital advice, only to be told "Charles is hopeless", the Daily Mail reported.
The Princess, who died in a 1997 car crash in Paris, also candidly discusses her battle with bulimia and her close relationship with protection officer Barry Mannakee.
During the sessions with voice coach Peter Settelen, Diana reveals that she confronted Charles about why Camilla Parker Bowles - now his wife the Duchess of Cornwall - was a part of his life.
Diana said: "I remember saying to my husband, 'Why? Why have you got this lady around?'
"And he said, 'Well, I refuse to be the Prince of Wales who never had a mistress.'
The Princess said at the height of her marital woe over Charles' affair she went to the Queen for help.
She said: "So I went to the top lady, sobbing and I said, 'What do I do? I'm coming to you, what do I do?'
"And she said, 'I don't know what you should do, Charles is hopeless'. And that was it, that was help."
Diana also discusses how she met Charles and reveals that the pair only spent time together on 13 occasions before they got married.
She said: "I was asked to stay with friends in Sussex and they said, ''Oh, the Prince of Wales is staying because he's playing polo,'' she recalls.
"I thought I hadn't seen him in ages. He had just broken up with his girlfriend and Earl Mountbatten had just been killed."
She revealed that she hadn't been that taken with the Prince on previous occasions, but this time her feelings changed.
Diana said: "I am quite impressed. He chatted me up, [he was all over me] like a bad rash; I thought...ehh [pulls back, pulls face].
"We were at a barbecue that night talking about Mountbatten and his girlfriend and I said, 'You must be so lonely.'
"I said, 'It's pathetic watching you walking up the aisle with Lord Mountbatten's coffin in front, ghastly. You need someone beside you...'
"Agggh. Wrong word! Whereupon he leapt upon me and started kissing me and everything and [waves arms] urrgh...You know, this is not what people do.
"Next day he said, 'You must come to Buckingham Palace with me, I have some work to do but you wouldn't mind sitting while I do my work.'
"I thought, 'Well, bugger it, I do mind sitting there while you do your work,' and I said that and it sort of lit up something in him, that someone answered back. So I was quite a challenge."
She added: "He wasn't consistent with his courting abilities.
"He'd ring me every day for a week, then wouldn't speak to me for three weeks. Very odd. I thought, 'Fine. Well, he knows where I am if he wants me.'
"The thrill when he used to ring up was so immense and intense. It would drive the other three girls in my flat crazy."
While behind palace doors the royal couple faced emotional struggles, Diana said that in public they were a 'good team'.
She said: "I used to get in the car with Charles and I used to blub in the car. There would be crowds everywhere and he would say, 'Now what's the matter?' I said, 'I can't be in this car.'
"He said, 'Why?' 'I can't be in this car, I don't feel safe.' I was neurotic almost but then when I got out of the car. [pulls face of calm]"
The Princess confessed she found solace with her married police protection officer Mr Mannakee, a relationship she suggested was not sexual, but in the tapes she reveals she considered fleeing the Royal Household to be with him.
She also claimed she had sex with her husband Charles "once every three weeks" but it fizzled out six or seven years before the tapes were made, around a few years after Prince Harry was born in 1984.
The Princess claimed the Duke of Edinburgh had told his son he could have an affair with Camilla - if his marriage had failed after a set period.
In the video recordings - aired in a US documentary 13 years ago but never screened in the UK - a relaxed and candid Diana said: 'My father-in-law said to my husband 'if your marriage doesn't work out, you can always go back to her after five years'.
'Which is exactly - I mean, for real I knew that it had happened after five (years) - I knew something was happening before that but the fifth year I had confirmation.'
The Princess hired Mr Settelen between 1992 and 1993 to help with her public speaking voice, following her collaboration with author Andrew Morton on a biography, and ahead of her bombshell Panorama interview in 1995.
The footage, captured at her private residence in Kensington Palace, shows Diana rehearsing her speaking voice but when discussing her personal life she is sat on a sofa, wearing a blouse, blazer and trousers.
Speaking about Mr Mannakee, Diana, said she fell "deeply in love" with the officer but when he later died she described the moment as the "biggest blow of my life".
At the time in the mid 1980s she was a mother caring for a young Prince William and Prince Harry and said: "I was quite happy to give all this up...just to go off and live with him. Can you believe it? And he kept saying he thought it was a good idea, too."
Rumours of an affair between Diana and Mr Mannakee spread throughout the Royal Household, and he was assigned to other duties - or "chucked out", as Diana put it. She said he later died in a motorcycle accident.
When her voice coach suggested there was virtually no sexual relations between her and Charles, Diana replied: "Once every three weeks and then it fizzled out about seven years ago, six years ago."
Diana also suggests that her failed marriage and the way she was treated by the royal family triggered an eating disorder.
She said: 'Everybody knew about the bulimia in the family. And they all blamed the failure of the marriage on the bulimia and it's taken them time to think differently.
"I said I was rejected, I didn't think I was good enough for this family, so I took it out on myself. I could have gone to alcohol. I could have been anorexic. I chose to hurt myself instead of hurting all of you."
A friend of Diana, Dr James Colhurst, who knew her from when she was 17 years old, said the Bulimia was having a worrying effect.
He told the Sunday Express: "You could see her fading physically. It was clear to all those who knew her that the bulimia was a reaction to the circumstances she found herself in."
Dr Colhurst added that Diana's "concerns about Camilla never stopped" and that she 'didn't know who to trust.'
He also claimed that the Princess was worried that the Palace would take her sons' away from her.
The Old Etonian said: "That was her worry, that she was going to lose the boys.
"Overriding, above everything else, that was the concern and that they were using a character rundown as a means of enabling that to happen."
The interview tapes were returned to Mr Settelen in 2004 after a lengthy dispute with Diana's family, headed by Earl Spencer, who said the footage belonged to them.
A batch of some 20 videos had been held by Scotland Yard after being seized in a January 2001 raid on ex-royal butler Paul Burrell's home.
The content of the tapes was regarded as so sensitive that the prosecution agreed not to use them in Mr Burrell's Old Bailey trial which collapsed in 2002.
The tapes were later sold to American broadcaster NBC for an undisclosed sum and excerpts were broadcast in 2004.
The Channel 4 documentary features some but not all of the footage recorded.
Diana: In Her Own Words will be screened on August 6.