"I believe you are well intentioned but surely you can see why it is a bad idea for me to meet with you," Assange wrote in January after Cumberbatch requested a meeting.
"By meeting with you I would validate this wretched film and endorse the talented, but debauched, performance that the script will force you to give.
"I cannot permit this film any claim to authenticity or truthfulness."
Assange asks the actor to reconsider his involvement in a project which "vilifies and marginalises a living political refugee" in favour of a corrupt and dangerous state.
The Australian argues films are powerful and insidious shapers of public perception and The Fifth Estate will smother the truth.
"As justification it will claim to be fiction but it is not fiction," he writes.
"It is distorted truth about living people doing battle with titanic opponents."
Assange insists there are dozens of positive books about WikiLeaks but Dreamworks based its script on the most toxic.
The Hollywood thriller received a long ovation at its world premiere at the Toronto Film Festival in early September.
Assange is masterfully played by Cumberbatch who speaks with an authentic Australian accent and is likely to be a firm favourite for an Oscar nomination.
Just days before the premier Assange told AAP he was hopeful the final version of the film - which he hadn't seen - might not be too bad.
He said at the time the publicity and having Cumberbatch speaking out about his organisation could even be "a good thing for the popularisation of Wikileaks".
"But people need to understand that this is a Hollywood movie and it has sections which are fictitious in order to increase the drama," the 42-year-old said.
Assange is holed up at Ecuador's embassy in London where he is avoiding extradition to Sweden over sexual assault allegations.
- AAP
