While Diana may have died in a car crash on Aug. 31, 1997, at the side of Dodi Al-Fayed, Burrell told ABC News' "Good Morning America Weekend Edition" in an interview that Al-Fayed wasn't her main romance toward the end of her life.
"The true love of her life, her soul mate, was a heart surgeon called Hasnet Khan," said Burrell, Diana's confidant and friend. "She met him while she was visiting someone in the Royal Brompton Hospital. The lift -- the elevator door -- was about to close and someone stuck their foot in it. The doors opened and she looked into his eyes and said, 'I knew there and then.' "
Burrell's book, "The Way We Were," is set to be published by Harper Collins Tuesday.
The tell-all book may be seen as an act of betrayal by the royal family, since Burrell's first book about his life with the princess, "A Royal Duty," didn't please the royal family.
Celebrity Couples of 2014 [PHOTOS]