Mohamed al Fayed is due to give evidence at the inquests into the deaths of Diana, Princess of Wales and his son, Dodi.
The Harrods owner has long maintained the princess and his son were murdered in a conspiracy involving the security services and the Duke of Edinburgh.
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Mr Al Fayed says his son told him in a phone call that he and Diana were soon to announce their engagement.
The couple died when their car crashed in Paris in 1997.
Driver Henri Paul was also killed.
BBC royal correspondent Peter Hunt says that, for four months, the inquest has looked at evidence behind many of Mr Al Fayed's claims.
At London's Royal Courts of Justice on Monday, the tycoon will personally tell the inquest jury of his theories surrounding the fatal crash.
Conspiracy theory
Our correspondent says that at the heart of Mr Al Fayed's case is a belief that a cordial friendship with the Royal Family soured once Diana and Dodi's relationship became public.
Mr Al Fayed claims they could not accept an Egyptian Muslim as stepfather to the future King of England - so a conspiracy, involving MI6 and Prince Philip, was hatched to murder the couple.
The coroner, Lord Justice Scott Baker, has already outlined Mr Al Fayed's allegations to jurors.
He told them: "It's his belief that a decision was taken to kill both Diana and Dodi. He places Prince Philip at the heart of the conspiracy.
"You will have to listen carefully to the witnesses you hear to see whether there is any evidence to support this assertion."
(BBC)
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