Channel 4 has defended the use of controversial video tapes of the late Diana, Princess of Wales, in a forthcoming documentary, describing the material as an "important historical source".

Featuring the footage in the documentary Diana: In Her Own Words has been strongly criticised by royal commentators and Rosa Monckton, one of Diana's closest friends, who said it was a "betrayal of her privacy".

But the broadcaster said the programme, due to be screened on Sunday, gave the Princess a voice and placed it "front and centre" in the run-up to the 20th anniversary of her death in a car crash in Paris on August 31 1997.

The documentary features Diana speaking candidly and informally about her upbringing, her courtship with the Prince of Wales, her troubled marriage and her public life.

Kensington Palace, the royal household of Diana's sons, the Duke of Cambridge and Prince Harry, has declined to comment about the documentary.

Channel 4 said in a statement: "The excerpts from the tapes recorded with Peter Settelen have never been shown before on British television and are an important historical source.

"We carefully considered all the material used in the documentary and, though the recordings were made in private, the subjects covered are a matter of public record and provide a unique insight into the preparations Diana undertook to gain a public voice and tell her own personal story, which culminated in her later interview for Panorama.

"This unique portrait of Diana gives her a voice and places it front and centre at a time when the nation will be reflecting on her life and death."

The tapes - recorded by Diana's speech coach Peter Settelen - have been screened in the US but never broadcast on British television, with some of the footage due to be shown for the first time.

Ms Monckton has said the tapes should be given to the princes and not broadcast.

She told the Guardian: "This doesn't belong in the public domain.

"It is a betrayal of her privacy and of the family's privacy. I certainly don't think they should be broadcast."

Diana 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 and reflecting at length about her life.

Dickie Arbiter, a former spokesman for the Queen, told Sky News: "It is absolutely shameful that these tapes have been made available.

"It seems that there is sort of grubby blood money running around.

"Channel 4 should really think deeply, not about ... their ratings figures but about the people and family who are still around who will find this very hurtful."

Ingrid Seward, editor of Majesty magazine, told the Sun: "I think it's remarkable any TV station would stoop that low. It really is lowest common denominator TV."

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, showing Diana talking of how Charles "leapt upon" her to kiss her at the start of her relationship.

She also told of running to the Queen after she became convinced that Charles had resumed his romance with Camilla Parker Bowles.

The lawyer who brokered the sale of the tapes, Marcus Rutherford, said while the recordings were intended to be private, the privacy had been lost after they were seen by police and the Spencer family.

But speaking to Stephen Nolan on Radio 5 Live, he said: "I don't think it's right to say that now, 20 years later after her death, they remain private any more.

"What is clear beyond any measure of doubt is that at the time she recorded these tapes and the time she was going through her separation from Prince Charles, she wanted the world to know what she was going through.

"She is not talking to a priest, she's not talking to a lawyer, she's not talking to a doctor or a therapist, she is talking to Peter openly and candidly and to be honest people who want to watch the film need to take a view for themselves."