Diana documentary will show controversial video tapes by her voice coach

Ahead of the 20th anniversary of the Princess's death, the film includes Diana speaking candidly and informally about her upbringing, her courtship with the Prince of Wales, her troubled marriage and her public life.

The tapes - recorded by Diana's speech coach Peter Settelen - have never before been broadcast on British television - and some of the footage is being shown for the first time.

The announcement that the video tapes are to be aired in the UK comes as ITV prepares to screen its own Diana documentary on Monday evening.

This features the Princess's sons the Duke of Cambridge and Prince Harry opening up about their loss, including their remorse at their final, brief phone call with their mother.

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.

Ralph Lee, Channel 4's head of factual commissioned Kaboom Film & TV to produce Diana: In Her Own Words with award-winning director Kevin Sim.

Lee said the documentary was an "important contribution to the historical record".

"The tapes, which show a relaxed and off-duty Diana, are hugely illuminating about her personality, humour and charm," he said.

"Combined with historical context and interviews with her closest confidants, this film provides a nuanced, multi-layered portrait of the most famous woman in the world and a mother who has shaped the future line of the Royal Family.

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

"It is her account of events both private and public and is an important contribution to the historical record."

Others who feature in the film include the Princess's long-term friend of more than 30 years Dr James Colthurst and her ballet teacher Anne Allan, who are speaking on the record for the first time.

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.

"I went to the top lady and I'm sobbing. And I said, 'What do I do?' (The Queen) said, 'I don't know - Charles is hopeless'."

Other admissions from the Princess included how she was deeply in love with a royal protection officer - presumed to be Barry Mannakee.

The bodyguard was killed in a motorbike accident and Diana revealed she believed he was "bumped off" because of their affair.

In 2004, the screening was met with criticism amid calls for the tapes not to be shown for at least 100 years

Sir Teddy Taylor, then Tory MP for Rochford and Southend East, appealed to broadcasters' "sense of decency" not to air the tapes while Diana's sons were still alive.

The Channel 4 film will be shown in early August - the same month as the 20th anniversary of Diana's death in a car crash in Paris on August 31 1997.