Lord Stevens, the former Metropolitan police commissioner who is leading the investigation into the Paris car crash, is reexamining forensic tests carried out on the blood of Henri Paul, the chauffeur who died along with Diana and Dodi Fayed, her boyfriend, in 1997.

The accuracy of the tests is crucial to the conclusion of the Stevens inquiry, which was ordered by the royal coroner two years ago in preparation for the inquest into her death, expected this year.

French police concluded after a lengthy investigation that the crash was an accident, caused by Paul driving the Mercedes in which Diana was travelling at high speed away from paparazzi while he was under the influence of drink and drugs. The blood tests, supposedly taken from Paul soon after he died, were central to their findings.

A source close to the inquiry said yesterday: “We are re-examining the circumstances surrounding the taking of the blood samples. Stevens is utterly determined to get to the bottom of this.”

Sources also say, however, that Stevens continues to take the view that the crash was an accident despite a possible mix-up over the blood samples.

Mohamed al-Fayed, the Harrods tycoon and Dodi’s father, has claimed the blood samples were deliberately tampered with by the authorities.

He has said the chauffeur was sober at the time of the crash in the Pont d’Alma tunnel in central Paris. He believes the princess and his son were murdered by British intelligence because the Establishment, including the Duke of Edinburgh, decided Diana must not marry Dodi, a Muslim.

An essential feature of the conspiracy theory is that Diana was pregnant and the couple were due to marry. Friends of the princess and inquiry sources discount both suggestions.

Stevens has also decided to re-interview at least two MI6 officers who were stationed in Paris at the time of the crash. He is looking into suggestions that MI6 agents visited the morgue where the bodies of Diana, Dodi and Paul were kept after the crash. There have been unsubstantiated claims that blood samples from Paul were swapped in the morgue with those of a suicide victim who had been drinking heavily, to make it appear that the chauffeur was drunk.

The latest flurry of speculation has been prompted by a recorded interview to be shown on GMTV today, in which Stevens said his inquiry was “far more complex than any of us thought”. He said new witnesses had been found and that some questions asked by al-Fayed were “right to be raised”.

• Lord Mishcon, the former solicitor for Diana, Princess of Wales has died at the age of 90, his family said yesterday.

  Mishcon, a Labour life peer who served as shadow lord chancellor from 1990-92, died at his home in Bayswater, west London, last Friday. One of the country’s most senior lawyers, he oversaw negotiations for Diana’s 1996 divorce from the Prince of Wales.

  His son Peter said: “He died at the end of a long life. He had been declining for a long time.”

  Born in 1915, Mischon founded a firm of solicitors in Brixton, south London, that eventually became Mishcon de Reya. He served in local government with Lambeth borough council, London county council and the Greater London council.

  Lord Falconer, the lord chancellor, said: “Lord Mishcon was a wise and decent man. He was a man who put every community of which he was a member before himself. The law, the Labour party and the Lords have lost a very big man.”

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