In an interview with the Mirror, Jean said he believes the royal and her partner were killed in a bid to stop the future king of Britain’s mother marrying a Muslim.
“Inside Scotland Yard, some believe there was a secret plot to kill Diana,” he said.
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He alleged the couple’s chauffeur was “collateral damage” in an establishment plot.
Several enquiries have ruled that Diana and Dodi died in the accident in Paris when driver Henri crashed their car while drunk.
However, Jean, who says he has seen classified information regarding the tragedy, said he doesn’t believe this.
“Diana was killed and my son was killed. I believe they were both murdered. My son was simplycollateral damage of a plot to kill Diana and they killed him as well.
“I am 100 per cent sure he was not involved in this plot. He was too honourable and too honest. The English police came here to visit me and sat exactly where you are sitting now.
“Even inside Scotland Yard there are two sides. One believes there was a secret plot to kill Diana, the other believes it was a genuine accident.
“Even today there are too many classified secrets with this incident.
“I have no real hope to know what really happened. Perhaps it will be known in 30 or 50 years. But I would really like to know before I die. But I don’t think it will happen.”
He added that he didn’t believe that his son, 41, was drunk at the time of the crash, saying that his blood sample could have “easily” been tampered with or swapped.
“He was not an alcoholic. He was not a drug addict. That’s just lies,” he said.
Dodi’s father Mohamed Al-Fayed has long claimed that the crash was a result of a conspiracy orchestrated by MI6.
His claims were dismissed by a French judicial investigation and by the Metropolitan Police Service inquiry.
In April 2008, a jury at the inquest concluded that Diana and Fayed were the victims of an “unlawful killing” by the “grossly negligent” chauffeur Paul and the drivers of pursuing vehicles.
In found that additional factors were “the impairment of the judgment of the driver of the Mercedes through alcohol”.