Princes William and Harry appealed last night for an end to the speculation surrounding the death of their mother after a three-year investigation demolished claims of a conspiracy to murder her. But the official police report into the deaths disclosed that Dodi Fayed had probably been intending to ask Diana, Princess of Wales, to marry him on the night she died.

The 832-page report also discloses details of the Princess’s state of mind at her marriage collapse, including her fears that both she and Camilla Parker Bowles were to be “put aside” to enable the Prince of Wales to marry a family friend.

It reveals that the Princess believed that the Duke of Edinburgh wanted her dead and that she would end up “chopped” like Mary, Queen of Scots, or declared unbalanced after a car crash in which she would be seriously injured.

The findings of the £3.6 million inquiry headed by Lord Stevens of Kirkwhelpington, the former Metropolitan Police Commissioner, were unveiled in Westminster. His team of twelve police officers spent almost three years investigating theories surrounding the death of the Princess, 36, and her boyfriend, Dodi Fayed, 42, in a car crash in Paris in August 1997.

Lord Stevens declared that allegations that the Princess was murdered were unfounded and he found nothing to justify further inquiries with members of the Royal Family. He said that the couple and their chauffeur died as a result of a “tragic accident” rather than a murder plot. His demolition of conspiracy theories was angrily dismissed by the Harrods owner, Mohamed Al Fayed, who called the report a “cover-up” and accused MI6 and a senior member of the Royal Family of being involved in a conspiracy to assassinate them.

Mr Al Fayed, who is understood to have been spending millions of pounds a year on pursuing his claims of a conspiracy, pledged to spend his fortune to get at the truth.

Lord Stevens’s report dismissed claims of a conspiracy involving the intelligence services to murder the Princess. It also rejected suggestions that she was pregnant when she died. Her friends and family told police that she had no intention of getting engaged to Fayed, although the report suggests that he was preparing to propose to her with a ring from a “Tell me yes” range sold by a Paris jeweller. The Princess had told friends in phone calls from her Mediterranean cruise with Fayed that she needed marriage “like a rash on my face”.

The report adds that Prince William had no knowledge of any plans for his mother to get engaged.

The inquiry concludes that, rather than there being a conspiracy, the couple and their chauffeur Henri Paul died in a late-night accident in the Pont de l’Alma underpass in Paris. Mr Paul was drunk and drove too fast as he tried to outrun photographers.

He had an alcohol level about twice the British drink-drive limit as he drove the Mercedes S280 through the underpass at between 61mph and 63mph, twice the speed limit. “Our conclusion is that, on all the evidence available at this time, there was no conspiracy to murder any of the occupants of the car. This was a tragic accident,” Lord Stevens said.

“I very much hope that all the work we have done, and the publication of this report, will help to bring some closure to all who continue to mourn the deaths of Diana, Princess of Wales, Dodi Fayed and Henri Paul.”

The next stage in the investigation into the Princess’s death will be an inquest next year.