Cumberbatch plays Assange in the upcoming DreamWorks drama.
The actor reached out to Assange before he began work on the film, but Assange wrote back to Cumberbatch last January, declining to help with the project, Variety said in a story accompanying the letter, which it published Wednesday.
"I believe you are a good person, but I do not believe that this film is a good film," Assange wrote in the letter. "I do not believe it is going to be positive for me or the people I care about. I believe that it is going to be overwhelmingly negative for me and the people I care about. It is based on a deceitful book by someone who has a vendetta against me and my organization."
The film is an adaptation of Daniel Domscheit-Berg's "Inside WikiLeaks: My Time with Julian Assange and the World's Most Dangerous Website," a book Assange has denounced as inaccurate.
"Your skills play into the hands of people who are out to remove me and WikiLeaks from the world. ... I believe that you should reconsider your involvement in this enterprise," Assange told Cumberbatch in his missive.