A documentary about Princess Diana was British network Channel 4's highest-rating show of 2017, nearly 20 years after she died. "Diana: In Her Own Words" was also Channel 4's highest-rating documentary since 2014, in spite of the controversy surrounding it.

The special was based on taped conversations of Diana made by her voice coach Peter Settelen in 1992 and 1993.

Some of Diana's friends and supporters opposed the film's release, saying that it would upset her sons, Prince William and Prince Harry, reports Variety. The documentary includes Diana sharing intimate details about her personal life, married life and even her sex life.

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But Charles Furneaux of Kaboom Film & TV, who produced the program, said, "It's no different than any historical film. You get the best source material available. You often see the use of video that was not intended for public use."

The show aired Sunday night and averaged 3.5 million viewers, peaking at 4.1 million viewers. A one-hour version of the show called "Diana -- Her Story" will air on PBS in the U.S. on Aug. 22. A different show called "Diana: In Her Own Words," from 1895 Films, will air on National Geographic in the U.S. and abroad. Aug. 31 marks the 20th anniversary of her death.

Channel 4 will air another documentary about Diana called "Princess Diana's 'Wicked' Stepmother," which will focus on Diana's tumultuous relationship with her stepmother and subsequent reconciliation. The show airs on Thursday.