London: Recordings of Princess Diana speaking candidly about her sex life with Prince Charles and her sorrow at the spectacular collapse of her marriage will be broadcast on British television on Sunday.

Diana, first wife of the heir to the British throne, died aged 36 in Paris on Aug. 31, 1997, after the limousine carrying her and lover Dodi al-Fayed crashed in a tunnel as it tried to escape paparazzi who were chasing them on motorbikes.

Diana was just 19 when she was betrothed to Charles in 1981, but the marriage broke down. Diana blamed Camilla Parker Bowles, Charles's lover who later became his wife, for the collapse of the marriage.

In a documentary planned to coincide with the 20th anniversary of Diana's death, broadcaster Channel 4 will show footage of her speaking about her marriage during privately recorded sessions with an adviser on public speaking.

The footage includes Diana speaking about her sex life with Charles.

"It was just so odd. I don’t know there was no requirement for it from his case," Diana says. "Sort of once every three weeks."

"And then I followed a pattern, he used to see his lady once every three weeks before we got married," she said.

Diana also relates how she approached Queen Elizabeth to discuss the failing marriage.

"She said, ‘I don’t know what you should do, Charles is hopeless’. And that was it," Diana said. She also said Charles's father, Prince Philip, had given him permission to have an affair if the marriage did not work.

The footage of Diana was filmed at Kensington Palace in 1992 and 1993 by speech coach Peter Settelen. After a lengthy court battle, the tapes were handed to Settelen.

Segments of the tapes were aired in the United States by channel NBC. In 2007, British broadcaster the BBC also tried to use the tapes for a documentary, but the film was scrapped.

Marcus Rutherford, Settelen's lawyer, said his client had tried to keep the tapes private but that this privacy was broken during the court battle.

"Peter was not her priest, doctor, therapist or lawyer," Rutherford said in an emailed statement, defending his subsequent decision to release them.

"It is very clear that Diana herself wanted the world at large to know about the causes of the problems in her marriage," he said.

Friends of Diana have criticised Channel 4 for what they said was a violation of privacy.