The Prince of Wales is expected to brief his sons, William and Harry, on exactly how their mother died before Lord Stevens unveils the findings of his investigation into her fatal Paris car crash.

Prince Charles and Mohamed al-Fayed, whose son Dodi, was also killed in the crash, will receive a copy of the findings on Wednesday, a day before the report is published.

The report compiled by Lord Stevens, the former Metropolitan police commissioner, is expected to quash the last of the conspiracy theories surrounding the car crash that killed Diana, Princess of Wales, nine years ago in the Pont d’Alma tunnel, Paris. New DNA tests appear to have confirmed that Henri Paul, the French chauffeur, was more than three times over the French drink-driving limit when he crashed the Mercedes carrying Diana and Dodi Fayed.

Claims that Diana was pregnant and planning to marry Dodi are also dismissed by friends and experts in a BBC documentary to be broadcast this evening.

The new findings will be a blow to al-Fayed, who has claimed that Paul was portrayed as a drunk to cover up an elaborate Establishment plot in which Diana was murdered to stop her marrying Dodi, a Muslim.

Fayed’s spokesman issued a statement yesterday disputing the new DNA claims and accused the BBC of being “very naive or complicit in the ongoing cover-up”.

Lord Stevens’s two-year inquiry, costing £2m, is expected to draw similar conclusions to those of French investigators in September 1999. Paul, who died at the wheel, was driving at excessive speed while under the influence of drink and prescription drugs.

Last week Fayed did have one piece of good news when he won his legal battle to ensure the preliminary hearings ahead of Diana’s inquest will be held in public.

They will decide in January whether a jury should sit on the full inquest later in the year. It has not yet been determined whether Diana and Dodi’s inquests will be held jointly.

But claims that the crash was an MI6 plot are likely to get short shrift. Martine Monteil, the police chief who led the French investigation, described such ill-informed speculation as “irritating”.

She said there was “not a shred of doubt” that the crash was caused by Paul’s high-speed drunk-driving. Fayed has fought a long legal battle attempting to prove that Paul’s blood samples had been deliberately switched in the mortuary to smear him.

But Monteil said: “Blood samples were taken immediately after the crash, and a DNA test was carried out on the same blood samples as recently as last year and compared with his own parents’ DNA, proving it was his blood.”

Suggestions that Diana was planning marriage and was pregnant are also dismissed by friends and experts interviewed for BBC2’s The Conspiracy Files.

Rosa Monckton, Diana’s close friend, said a ring given to the princess by Dodi was merely a gift and did not signify that the couple were engaged. Professor Andre Lienhart, who reviewed the emergency services’ response for the French investigation, said: “The autopsy showed that she was not pregnant.”

A spokesman for Fayed, owner of Harrods department store, said the new DNA evidence was based on “a false understanding” of the analysis of Paul’s blood.

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