Scotland Yard chiefs face new concerns over their handling of a note detailing Princess Diana‘s fears she would be killed in a staged car accident.

Diana voiced her fears to her lawyer, Lord Mishcon, in October 1995. She died in a car crash in Paris two years later alongside Dodi Fayed and driver Henri Paul.

The mystery surrounding the note is revisited in a four-part documentary – Investigating Diana: Death In Paris – beginning tonight on Channel 4 to mark the 25th anniversary of the tragedy.

After Diana’s death, Lord Mishcon passed his contemporaneous typed account of their meeting to senior Metropolitan Police officers who put it in a safe. But the note was not passed to French authorities investigating her crash for six years.

Diana’s brother and sisters learned of its existence only more than a decade after it was written. Princes William and Harry were also left in the dark for a long time.

There have even been suggestions of a mysterious addition to the note, which it has been claimed was designed to give cover to the fact the original note was not released earlier.

According to John Morgan, author of How They Murdered Princess Diana, a second page was written in pen on a different pad and on different dates.

At his meeting with police chiefs the month after Diana’s death, Lord Mishcon read his note aloud to stress its importance. He told officers that it recorded Diana saying that ‘efforts would be made if not to get rid of her by some accident in her car, such as a pre-prepared brake failure… at least to see that she was so injured or damaged as to be declared unbalanced [in her mind]’.

Michael Mansfield, a lawyer who represented Fayed’s father Mohamed Al Fayed, tells tonight’s programme: ‘The note is important because it’s equivalent to somebody’s premonition. If you were a police officer investigating it, you want to hand the account over to the French. They didn’t do that. They stick it in the safe and they don’t reveal it.’

The Mishcon Note, as it became known, may well feature in Prince Harry’s controversial memoir, due out later this year.

Harry is said to be ‘intensely focused’ on investigating his mother’s final hours. Neither he nor his brother were aware of key details for almost a decade, according to the documentary.

In 2006, former Met commissioner Lord Stevens led Operation Paget, which investigated conspiracy theories surrounding the accident. Later that year, he briefed William, then 24, and Harry, 22, at Kensington Palace for 90 minutes about his report’s detailed findings.

Until then, the Princes had only ‘limited knowledge’ of the accident. Lord Stevens said he fielded ‘very pertinent questions’ from the Princes, saying later it was a ‘difficult session for them’.

‘I was in possession of the facts of what had taken place, from the beginning of the problem outside the Ritz with the car, to the death and bringing back the body,’ he said. ‘They wanted to know the circumstances of the death, what had happened to their mother, in every aspect. Some questions were in detail – which I answered, because they hadn’t been told of the circumstances.’

Operation Paget concluded that Diana’s death was a ‘tragic accident’. There was, Lord Stevens concluded, ‘no evidence’ of a murder conspiracy or a cover-up by MI6.

He blamed the drunken driver, Henri Paul, for the high-speed accident, which happened as Diana and Dodi drove from a dinner at the Ritz hotel to an apartment.

Lord Mishcon died aged 90 before the inquest but had given a statement to Operation Paget, which is how existence of the Mishcon Note emerged and how Diana’s sister Sarah first heard of it.

Sandra Davis, a lawyer working for Mishcon who was at the Diana meeting, told the inquest: ‘Lord Mishcon was concerned to take the note… because he thought that it was important that the police knew that he had made it and that she [Diana] had said what she had said.’

A record was made by police of their September 1997 meeting with Lord Mishcon. It concluded that ‘if it ever appeared’ there were some suspicious factors to the death crash, the lawyer or his firm would be contacted by Scotland Yard.

But when the same record appeared at the inquest years later, it had a second page attached with wording that backed up the police decision to keep the note secret.

The Mishcon Note came to light when Diana’s butler, Paul Burrell, revealed that he possessed a similar letter from Diana predicting her death. Lord Mishcon tried to contact Lord Stevens and police consulted lawyers about what to do with the Mishcon Note in the safe.

It eventually arrived on the coroner’s desk from Scotland Yard on December 30, 2003. An inquest into Diana’s death was opened a week later, then adjourned. It concluded in 2008 that she was ‘unlawfully killed’, partly because of the ‘gross negligence’ of the driver.