The report also said Diana was not pregnant, that she was not engaged to marry Dodi Fayed, and that their chauffeur was drunk and driving at more than 60 mph — twice the speed limit — when their car crashed while being chased by photographers.
The inquiry, which largely confirmed previous findings by French investigators, also said there was no reason to suspect the involvement of the royal family in the death of Prince Charles' former wife.
"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," said Lord John Stevens, former chief of the Metropolitan Police, who led the investigation of the deaths of Diana, 36, and Fayed, 42.
"There was no conspiracy, and no cover-up," Stevens added.
The couple was killed along with chauffeur Henri Paul when their Mercedes crashed in the Pont d'Alma tunnel in Paris on Aug. 31, 1997, while being chased by media photographers. Bodyguard Trevor Rees-Jones was seriously injured.
Paul was drunk, with a blood-alcohol level twice the British legal limit, and driving at twice the local speed limit before the crash, Stevens said.
CBS News correspondent Sheila MacVicar says Stevens' report does imply that the photographers present that night did share some of the blame for the accident.
Stevens said the photographers had prompted Diana and Fayed to change travel plans before their death.
The French courts acquitted nine photographers of manslaughter, but this result may re-open questions about exactly what role they did play that night, MacVicar says.
Fayed's father, Mohammed al Fayed, rejected the report, calling it "garbage."
He has alleged that the couple was killed as a result of a plot by the establishment, including British intelligence agencies and Prince Philip, her former father-in-law.
"I am certain, 100 percent, that a leading member of the royal family have planned that and the whole plot, being executed, in his order with the help of members of MI6," al Fayed, owner of Harrods department store, said at a news conference after the report was released.
"I am the father who lost his son and close friend, Princess Diana. Nobody have any right just to predict and spreading rumor, displaying things which is not completely real," al Fayed said, referring to Stevens.
Al Fayed said Diana "conveyed to me all her suffering, all the devastation of threats she have in her life in the last 20 years she was living in the royal family environment."
Contradicting long-standing rumors, family and friends denied in interviews that Diana was about to marry Fayed, and Diana was not pregnant, Stevens said.
"From the evidence of her close friends and associates, she was not engaged and not about to get engaged," Stevens said.