Diana’s privacy over videos of her speaking candidly about her troubled marriage was breached long ago, and not by the voice coach who made them, his lawyer has said.

As friends and family of the late princess condemned Channel 4’s forthcoming documentary based on the tapes, Diana: In Her Own Words, Marcus Rutherford, lawyer for Peter Settelen, defended the decision, saying: He was not her priest, doctor, therapist or lawyer.

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.

They were made in her Kensington Palace apartment between 1992 and 1993 to help her with public speaking.

Channel 4 said the tapes, which will be broadcast on Sunday, are an important historical source.

Kensington Palace said neither of Diana’s sons, William or Harry, would be making any public comment.

Extracts from the tapes first aired on the American NBC network in 2004, but have not previously been broadcast in the UK.

Settelen holds the copyright to the tapes, which were returned to him following a legal battle with the princess’s estate led by the Spencer family. The tapes were seized by Scotland Yard in a 2001 raid on the home of a former royal butler, Paul Burrell. They were regarded as so sensitive at the time of Burrell’s collapsed Old Bailey trial in 2002 that the prosecution agreed not to use them.

Prince Charles and Lady Diana Spencer in May 1981, before their marriage one of only 13 premarital meetings, according to Diana. Rutherford said the debate over their broadcast was valid.

But it is very clear that Diana herself wanted the world at large to know about the causes of the problems in her marriage: quite apart from the Settelen tapes, she spoke about them on Panorama and extensively on tape to Andrew Morton [a biographer of Diana] in the full knowledge they would be published, and presumably fully aware of the distress it would cause her family and the royal family.

At the time, the general public overwhelmingly took her side, and whether or not you believe that she should have been so open is really for the viewers to form their own opinions.

Peter was not her priest, doctor, therapist or lawyer. Diana was not disclosing confidences to a lover and the tapes were not secretly recorded.

A teenage Lady Diana Spencer with her pet pony Scuffle in 1974.

He added Settelen had tried very hard” to keep the tapes private. Despite all his efforts, over four years of legal battles, such privacy as existed was broken not by Peter, but by others, including the Metropolitan police.

A whole industry of people had been prepared to give their take on Diana. Peter was eventually persuaded that he should allow the public to see and hear exactly what Diana had to say, rather than have her message spun by others who would frankly prefer to silence her voice entirely.

Rutherford added much of the recent media outrage over the material would be more compelling if very many of their number had not also made strenuous efforts over the years to secure it exclusively for themselves.

Dickie Arbiter, a former spokesman for the Queen, told Sky News that airing the tapes was absolutely shameful and it seemed there was grubby blood money running around. A biographer of Diana, Ingrid Seward, has described it as lowest common denominator TV, while writer Penny Junor accused Settelen of prurient money making.

Diana’s brother, Charles Spencer, had reportedly tried to stop the documentary and Rosa Monckton, a close friend of Diana’s, said it was a betrayal of her privacy and of the family’s privacy.

The BBC reportedly decided to shelve a documentary using the tapes in 2007, the year of the 10th anniversary of her death.

Channel 4 said: 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.

In the recordings Diana talks of having met Charles just 13 times before they were married, and claims he was at first all over me like a rash then sex dwindled to once every three weeks before fizzling out. She said that when she confronted Charles about his relationship with Camilla, the prince retorted that he refused to be the first Prince of Wales without a mistress.

When she turned to the top lady, the Queen, for advice, the monarch allegedly replied: I don’t know what you should so. Charles is hopeless. Diana also speaks of her infatuation with her married police protection officer, Barry Mannakee, who was moved from his job and later died in a road traffic accident.