Another user pointed out: "There are dozens of British actors that would be much more suitable. What are they thinking? I suspect that this one might get reversed."
Speaking on the plotline to news outlet Deadline, director Pablo Larrain said: "We decided to get into a story about identity, and around how a woman decides somehow, not to be the Queen."
Diana and Charles wed in 1981 but separated in 1992 soon after the breakdown of their relationship became public knowledge.
The details of their marital difficulties became increasingly publicised, and the marriage ended in divorce in 1996.
Stewart is best known for playing Bella Swan in the successful Twilight franchise and most recently was one of three angels in a reboot of Charlie's Angels.
Larrain, who previously directed Jackie, 2016's biopic of Jackie Kennedy, says Stewart is the perfect fit for the royal role.
"Kristin can be many things, and she can be very mysterious and very fragile an ultimately very strong as well, which is what we need," he told Deadline.
The Princess of Wales has been portrayed on screen before - in 2013 Naomi Watts played her in the critically panned 2013 film, Diana.
Kristen Stewart will portray Princess Diana in the new film 'Spencer'. Photos / Getty Images
Diana told the story of the last two years of Diana's life including her romances with heart surgeon Hasnat Khan and business mogul Dodi Fayed.
It had been expected to be an Oscars contender but instead flopped at the box office.
Critics dubbed the film "car crash cinema" and wrote "Wesley Snipes in a blonde wig would be more convincing than Watts' performance".
