Mohamed Fayed reportedly believes his son Dodi and Princess Diana were set to announce their engagement shortly before they died in a car crash.
The 88-year-old businessman lost his son in the car accident in Paris, France, in August 1997, and reportedly felt the incident stopped the duo from tying the knot.
A source said: "Mohamed believes they were in love and were going to announce their engagement in London the day after the tragedy.
"He will never get over the death of his son, or that of the Princess - because of the love he had for both."
Mohamed still spends a significant amount of time sat beside his son's body in the grounds of his mansion, Barrow Green Court, near Oxted, South-East England.
Another insider told The Sun newspaper: "He often spends hours on end sitting with Dodi. After 20 years he still misses him terribly.
"And when he is in London he will take time out to visit Dodi's apartment. Not one thing inside has changed since Dodi died. He will not allow anyone to move anything.
"The apartment is cleaned but it is still exactly the same as when Dodi and Diana used to spend time there.
"They used to sit on the floor having takeaway meals. She and Dodi were mad on films.
"He had a wall filled with VHS tapes. Any moment they had together, they went there. They'd watch TV and hang out like teenagers."
What's more, the insider said the relationship represented the chance for Princess Diana to relive her youth, following her divorce from Prince Charles.
The source explained: "In a way, the Princess was having her youth again because she had been in the Royal Family for 15 years.
"This was her first affair as a free woman. She didn't hide the relationship or sneak away. She'd been divorced since the year before."