New recordings of her interview with voice coach Peter Settelen reveal Diana's sex life with her husband Prince Charles.
Ahead of the 20th anniversary of her death in a Paris vehicle crash, Channel 4 will broadcast Diana: In Her Own Words on August 6, despite a report in The Mail on Sunday newspaper that her brother Earl Charles Spencer is unhappy with the decision.
The documentary airs August 6 on Channel 4.
In the trailer for the explosive documentary, Diana says she only met Charles 13 times before they were married.
As well as describing the 180-minute edit as a demonstration of Diana's "backbone of steel", Jephson says: "What you get beautifully through those tapes is her sense of spontaneity and fun".
The broadcaster has come under fierce criticism over its decision ahead of the 20th anniversary of Diana's death to play excerpts from the tapes it has acquired for an undisclosed sum from Settelen, an actor turned voice coach.
In the video recordings - aired in a U.S. documentary 13 years ago but never screened in the United Kingdom - 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".
It is understood the tapes show Diana going off-script and recalling intimate details of her courtship with Charles.
The tribute also includes a case of cassette tapes, featuring albums by artists such as George Michael, Diana Ross and Elton John, who famously sang at her funeral.
Diana said in the recordings that the couple had sex "once every three weeks" but that their sex life dried up long before the tapes were recorded in 1992-93. One would assume that these conversations were meant to help with her many public speeches on land mines and AIDS.
Ingrid Seward, author of "Diana: The Last Words", said: "This is using Diana to make money and that is what killed her in the end".
Diana also claimed Prince Philip told Charles that if his marriage to Diana didn't work out, he could go back to Camilla "after five years". "Combined with historical context and interviews with her closest confidants, this film provides a nuanced, multilayered portrait of the most famous woman in the world and a mother who has shaped the future line of the royal family", Channel 4 told the Mirror.
An upcoming documentary set to air on Channel 4 features the tapes, which originally aired on US television in 2004. She then would talk about how the royal family disapproved of the unconsummated affair because "they didn't have any proof".
The ballet teacher also noticed that Diana had lost weight and the Princess told her that she had bulimia.
She said: 'This doesn't belong in the public domain.
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