Princess Diana is reported to have described her wedding as “the worst day of my life” in explosive new video tapes set to be screened to mark the 20th anniversary of her death in a car accident in Paris in August 1997.

The previously unseen footage, which shows the Princess talking to her friend and voice coach, Peter Settelen, forms the basis of a new Channel 4 documentary about her life. They were recorded during several sessions at Kensington Palace in 1992 and 1993.

In the footage, Princess Diana says she had only met Prince Charles 13 times before they married, according to The Telegraph.

“He'd ring me up every day for a week and then he wouldn't speak to me for three weeks,” she said. “Very odd.”

"And the thrill when he used to ring up was so immense and intense."

Discussing her sex life with the Prince, she said there was “never a requirement for it from him” – something she believed was a result of his affair with Camilla Parker Bowles, who he later married.

"Once every three weeks about and I kept thinking it followed a pattern”, she said.

"He used to see his lady once every three weeks before we got married."

She later said her wedding was the “worst day of my life” and added: “If I could write my own script I would have my husband go away with his woman and never come back."

A Day That Shook The World: Charles and Diana separate Show all 10 1 /10 A Day That Shook The World: Charles and Diana separate A Day That Shook The World: Charles and Diana separate 24th February 1981: Charles, Prince of Wales, and Lady Diana Spencer at Buckingham Palace in London on the occasion of their engagement. Getty Images A Day That Shook The World: Charles and Diana separate Charles, Prince of Wales, and Diana, Princess of Wales on the banks of the River Dee in the grounds of Balmoral Castle, Scotland whilst on their honeymoon. Queen Victoria's husband, Prince Albert, purchased Balmoral Castle in 1846, and the small castle which stood in the 7,000 hectare wooded estate was redeveloped in the 1850s. Getty Images A Day That Shook The World: Charles and Diana separate London's national newspapers headline 21 December the report that Queen Elizabeth has sent a letter to both Prince Charles and the Princess Diana spelling out her 'desire for an early divorce.' Getty Images A Day That Shook The World: Charles and Diana separate Charles, Diana and William during a photocall in 1983 2006 Getty Images A Day That Shook The World: Charles and Diana separate Diana and Charles on the steps of St Mary's hospital with their newborn son William in June 1982 Getty Images A Day That Shook The World: Charles and Diana separate 29th July 1981: Members of the royal family on the balcony at Buckingham Palace following the wedding of Prince Charles and Princess Diana. Also present are Prince Andrew, Prince Edward, Queen Elizabeth the Queen Mother (1900 - 2002), Prince Philip and Queen Elizabeth II of Great Britain. Getty Images A Day That Shook The World: Charles and Diana separate Lady Diana, Princess of Wales and Charles, Prince of Wales are seen during their wedding at St Paul Cathedral in London 29 July 1981. Getty Images A Day That Shook The World: Charles and Diana separate May 1981: Charles, Prince of Wales, and Diana, Princess of Wales, in the grounds of Balmoral Castle, Scotland whilst on their honeymoon. Getty Images A Day That Shook The World: Charles and Diana separate March 1981: Prince Charles and his fiance Lady Diana Spencer Getty Images A Day That Shook The World: Charles and Diana separate Prince Charles, Princess Margaret and Lady Diana Spencer in 1981 Getty Images

Mr Settelen has now sold the rights to the footage, which was filmed during lessons he gave Princess Diana in order to help improve her public speaking. It was never intended to be broadcast, and the Princess used the sessions to talk openly about her love life and marriage to Prince Charles.

The tapes were the subject of a lengthy dispute between Mr Settelen and Princess Diana’s family but were eventually returned to the voice coach after being discovered by police in a raid on the house of Paul Burrell, the Princess’s former butler, in 2001.

Some parts of the tapes were screened by US network NBC in 2004 but much of their content has never been aired.

Freelance director Kevin Sim has been hired to make the Channel 4 documentary, which is titled “Diana: In Her Own Words”.

Mr Sim, who has described the tapes as “dynamite”, was commissioned by the BBC in 2007 to make a programme about the Princess that was later scrapped after fears it would be too controversial.

Ralph Lee, Channel 4’s deputy chief creative officer and head of factual, 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.