Using controversial video tapes of Diana speaking to her voice coach, the Channel 4 programme chronicles her relationship with those closest to her in her own words.
When she confronted Charles about why Camilla Parker Bowles - now his wife the Duchess of Cornwall - was a part of his life Diana said he replied: “Well, I refuse to be the only Prince of Wales who never had a mistress.”
Diana confessed she found solace with her married police protection officer Barry 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.”
Diana described how she approached the Queen, or “the top lady” as she called her, for advice, and in the footage she still appeared visibly dissatisfied with the response as she told the story.
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, and that was help.”
The Princess hired Peter 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.”
The 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.