Lee, 60, has revealed to The Sun that he believes the royal would still be here today, if he had been in the car with Diana.
The bodyguard explained that he 'drew straws' which resulted in him not accompanying Diana and Dodi Fayed that fateful night.
“It could have been me in that car,” he told the publication. “When I learned they were not wearing seatbelts in the crash I understood why they didn’t survive. That shouldn’t have happened,” he said.
“It was standard practice for the family to wear seatbelts. It was an order sent down from the boss, Dodi’s dad Mohamed Fayed. Dodi, in particular, hated wearing seatbelts and I always insisted on it,” he admitted.
The bodyguard was nicknamed ‘Rambo’ and was planning to join Diana and Dodi in America.
“She didn’t want to [go to America], but that was the only place she felt people weren’t having a go at her. It was probably her way of keeping sane, to get some respite,” Lee recalls.
Harry’s emotional tribute to Diana
This comes after Diana’s son, Prince Harry, paid tribute to his mother in an emotional speech.
While talking to young recipients at the Diana Awards in July, Harry told the young recipients he still thinks of his mother "every day".
"There isn't a day during the past two-and-a-half decades where I haven't thought about the mark she left, not only on me and my brother, but on all of our lives," he said.
"I see her legacy in all of you. I see her legacy in a Diana Award community that spans multiple generations. I see her legacy every time I meet with families, young people, and children from all corners of the world.
"And I see my mum's legacy when I look at my own children every single day."
Prince William did not take part in the ceremony and instead wrote letters to the winners, saying his mum would have been "so proud".
The Diana Awards honour people aged between nine and 25 for their social and humanitarian work.
Never miss a thing. Sign up to Yahoo Lifestyle’s daily newsletter.
Or if you have a story idea, email us at email@example.com.