Actress Kristen Stewart said the story of Diana and Prince Charles' failing marriage is "one of the saddest to exist ever" as she prepares to play the late royal in an upcoming film.

Stewart is preparing to play the Princess of Wales in Spencer, which will tell the story of a weekend in the early 1990s when Diana decided her marriage to Charles was over.

Stewart, 30, revealed production is scheduled to begin in mid-January and in the meantime, she is doing extensive research so she knows the princess "implicitly".

Kristen Stewart virtually attends the Chanel Womenswear Spring Summer 2021 held on October 06, 2020 in Paris, France

She told InStyle: "The accent is intimidating as all hell because people know that voice, and it's so, so distinct and particular. I'm working on it now and already have my dialect coach.

"In terms of research, I've gotten through two and a half biographies, and I'm finishing all the material before I actually go make the movie.

Spencer will be directed by Pablo Larrain and the script has been written by Peaky Blinders creator Steven Knight.