Mr al Fayed said he was "the only person who knows the truth" about the death of Diana and his son Dodi in 1997.
Former Metropolitan Police chief Lord Stevens was presenting the findings of his three-year investigation at a press conference in central London later today.
Lord Stevens is expected to rule out various conspiracy theories about the crash, including Mr al Fayed's claims that it was caused deliberately by the British establishment.
Mr al Fayed, who owns Harrods, told BBC Radio 4's Today programme: "How can I accept something really shocking?.
"I know deep in my heart that I'm the only person who knows the truth."
Conspiracy theories have alleged that the death of the couple in a car crash in Paris was murder.
But the inquiry is widely predicted to say that the Princess and Dodi died as a result of an accident caused by drunk chauffeur Henri Paul.
Mr al Fayed hit out at Lord Stevens - whose appointment he initially welcomed - saying the detective had spent £5 million of taxpayers' money "wasting my time".
He claimed Lord Stevens had "betrayed" him and the country, and "done all what the intelligence wanted him to do" - referring to previous claims that British spies were involved in the crash.
"I am the only person who was close to Diana and to my son," Mr al Fayed said.
"She had no family. She have friends that pretend they were her friends."
Asked what grounds he had for disputing the report's expected findings, he responded that "the evidence is made up just to make it convenient for the terrorist and the gangster who murdered my son".
In angry exchanges, Mr al Fayed accused Today presenter James Naughtie of having "an agenda".
"If there's a question about which of us has an agenda, I think it's you," the interviewer shot back.
Mr al Fayed said: "I'm not going to continue to talk to you if you're just going to, you know .... I'm the father who lost his son. I'm the one who knows everything."
Key witnesses in the investigation and figures in the media had been "bought" by MI6, the businessman alleged.
The Harrods boss believes the couple were murdered in a plot by the British establishment, saying the Princess was pregnant and they were about to announce their engagement.
He will give his official reaction at a press conference in offices at his Knightsbridge store this afternoon.
Neither Clarence House, the Prince of Wales's official residence, nor Scotland Yard is commenting ahead of the release of the report.
The 800-page dossier into the deaths will be published and also appear on the internet.
Lord Stevens will set out the main findings of the case, codenamed Operation Paget, in the Queen Elizabeth II Conference Centre in front of the world's media at noon.
He is expected to dismiss the conspiracy theories and there have also been suggestions he will criticise the paparazzi more harshly than the 1999 French investigation.
He will be joined by Deputy Commissioner of the Metropolitan Police Paul Stephenson and the senior investigating officer, Detective Chief Superintendent David Douglas.
Prince William and Prince Harry were briefed on the outcome of investigation yesterday.
The Prince of Wales was interviewed last year as part of the inquiry and was apparently asked in a one-to-one meeting with Lord Stevens if he ever plotted to assassinate his ex-wife.
Diana, 36, and 42-year-old Dodi, were killed when their Mercedes crashed in the Pont de l'Alma tunnel in Paris on August 31 1997.
The couple had been pursued by paparazzi photographers after leaving the Ritz Hotel for Mr Fayed's apartment.
Lord Stevens was asked to undertake the major inquiry when the inquest into Diana's death opened and adjourned in January 2004.
A two-year investigation in France blamed Mr Paul for losing control because he was high on a cocktail of drink and prescription drugs and driving too fast.
One of the conspiracy theories suggested the blood samples from the chauffeur were switched, a claim understood to have been disproved by new DNA tests.
Other revelations rumoured to be in the report include allegations that the US secret service was bugging the Princess's telephone conversations in the hours before she died and confirmation that she was not pregnant.
The inquiry, estimated to have cost more than £2.5 million, is said to bring together some 20,000 documents and 1,500 witness statements.
Forensic teams examined the mangled black Mercedes S280 in painstaking detail.
New witnesses were discovered and cutting edge computer technology used to put together a 3D model of the car crash.
Lord Stevens admitted the inquiry was "far more complex than any of us thought".
"We have new witnesses, we have new forensic evidence," he said.
Lady Butler-Sloss, who is now in charge of Diana's inquest, is due to resume the hearings in January 2007.
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