Mohamed al Fayed insisted today that Princess Diana and his son Dodi were murdered on the orders of a member of the Royal Family.
The Harrods owner branded Lord Stevens' three-year investigation into the August 1997 Paris crash as "garbage" and a "cover-up".
The senior royal and MI6 were in it together, according to Mr al Fayed.
He said: "I'm certain 100 per cent that a leading member of the Royal Family has planned that and that the whole plot was executed on his order with the help of members of MI6."
An annoyed Mr al Fayed said that Lord Stevens, formerly Britain's most senior policeman as Metropolitan Police Commissioner, is "just a mental case" if he tries to brush aside his concerns as simply the words of a grieving father.
Lord Stevens is simply a "tool for the establishment and the Royal Family and intelligence", he insisted.
And he vowed that even if it cost him his last penny to expose the truth behind his son and Princess Diana's deaths.
Mr al Fayed said: "I have had threats but I believe in God and if they want to hurt me or any member of my family, the world knows that the terrorists and the gangsters have taken my son away from me.
"God will help me, I'm sure, and with God's blessing I will uncover and show the whole world and this country that they have terrorists that come and execute any crime with their power in Government and high places in the Royal Family, they can cover up anything and they think that the public can be duped."
Mr al Fayed denied that his belief of a plot was a smokescreen for the fact that chauffeur Henri Paul, his employee, was drink-driving. He took to the wheel despite having alcohol levels twice over the British drink-drive limit, according to the inquiry. But Mr al Fayed claimed Mr Paul was working for French intelligence or MI6 and proof of this was the large sums of money in his bank account and the 2,000 French francs found in his pocket when he died. The £3.69 million spent on the Stevens inquiry was a total waste, according to Mr al Fayed.
He said: "I feel sorry for the taxpayers and the money that has been wasted on such garbage. "Whatever it's going to cost me, if it costs me the last penny in my purse, I'm not going to rest until I get the gangsters." Mr al Fayed repeatedly refused to accept Lord Stevens' wide-ranging conclusions but said he would "definitely" accept a verdict by an inquest jury if it heard all the evidence. Lord Stevens said Princess Diana was not pregnant but Mr al Fayed claimed: "They embalmed her because she was pregnant." He said that Lord Stevens had at first been quite sympathetic but upon realising this, MI6 stole his laptop, probably holding key information. Lord Stevens said that Princess Diana and Dodi were not engaged or about to get married but Mr al Fayed's spokesmen suggested this was not true.