The heart surgeon, 54, has said that the film, starring Naomi Watts and Naveen Andrews, is based entirely "on hypotheses and gossip" and has vowed never to watch it.
The interview with the Mail on Sunday comes just weeks before the release of Diana, based on Kate Snell's 2001 book Diana: Her Last Love.
A still image from the film showing the actors side by side at the London hospital where they met was enough to convince the heart surgeon it would entirely misrepresent their relationship, he said. "I don't see this movie doing well at all," said Hasnat, who was nicknamed "Mister Wonderful" by the late Princess.
"It is based on gossip and Diana's friends talking about a relationship that they didn't know much about, and some of my relatives who didn't know much about it either. It is all based on hypotheses and gossip."
"I could tell immediately those were never our mannerisms, with [my] hands folded behind the back and all that," he added. "You could tell from that picture that it is all just presumed about how we would behave with each other, and they have got it wrong. There wasn't any hierarchy in our relationship. She wasn't a princess and I wasn't a doctor."
(Princess Diana Princess Diana arriving at the Royal Brompton National Heart and Lung Hospital, where Khan worked)
Hasnat has also denied producer's claims that he gave the film, which will premieres in London's West End on September 5, "tacit acceptance," adding, "It is a complete lie. I have never given any approval."
Last month, Diana's close friend Jemima Khan revealed in an exclusive interview to Vanity Fair that the Princess was so "madly in love" with Hasnat Khan that she "wanted to marry him, even if that meant living in Pakistan."
In the interview titled, "The Grandmother Prince George Never Knew," Jemima says Diana sought her advice on her romance with the heart surgeon, whom she dated from 1995 until 1997, during fundraising trips to Lahore.
(Princess Diana and Jemima Khan pictured in Pakistan)
Jemima, who at the time was married to Hasnat's distant cousin Imran Khan, said that the Princess had "wanted to know how hard it had been for me to adapt to life in Pakistan.
"(She) came to visit me twice in Pakistan to help fundraise for Imran's hospital, but both times she also went to meet his family secretly to discuss the possibility of marriage to Hasnat," she revealed.
Photos: Getty Images
A version of this article appeared at hellomagazine.com with the headline "Princess Diana's former lover Hasnat Khan says new film is based on gossip."