WikiLeaks has posted what it claims is a "mature" version of the screenplay for Julian Assange film The Fifth Estate, accompanied by a memo which labels the film "irresponsible, counterproductive and harmful".

The site describes Bill Condon's drama, which stars Benedict Cumberbatch as Assange, as "a work of fiction masquerading as fact", adding: "Most of the events depicted never happened, or the people shown were not involved in them."

WikiLeaks' 4,000-word missive also determines to pick apart what it claims is the film's central argument: that the site's release of classified US State Department documents in 2010 exposed and potentially endangered more than 2,000 informants worldwide. The role of Daniel Domscheit-Berg, which the film suggests was once Assange's right-hand man, is downplayed, and the memo seeks to lampoon suggestions that the site's founder dyes his hair white.

After posting the leaked script, WikiLeaks tweeted: "As WikiLeaks was never consulted about the Dreamworks/Disney film on us, we've given our advice for free: It's bad."

The Fifth Estate, which debuted at the Toronto film festival earlier this month, is partly based on Domscheit-Berg's own book Inside WikiLeaks: My Time with Julian Assange at the World's Most Dangerous Website, as well as Guardian writers David Leigh and Luke Harding's WikiLeaks: Inside Julian Assange's War on Secrecy. Condon has said his film hopes to "explore the complexities and challenges of transparency in the information age and, we hope, enliven and enrich the conversations WikiLeaks has already provoked."

However, the site's memo cautioned: "This film does not occur in a historical vacuum, but appears in the context of ongoing efforts to bring a criminal prosecution against WikiLeaks and Julian Assange for exposing the activities of the Pentagon and the US State Department."

Assange has been outspoken in his opposition to the film, which he has described as a "massive propaganda attack" in January. The WikiLeaks founder remains holed up in the Ecuadorian embassy in London, where he claimed political asylum in June last year to avoid extradition to Sweden to face sex-crime accusations. His lawyer has argued that his client would not receive a fair trial, and Assange also fears he could face onward extradition to the United States whether he is found guilty or not.

The political activist fell out with The Guardian and other former media partners in September 2011 after WikiLeaks published its full archive of 251,000 secret US diplomatic cables, without redactions, in a move which critics said could expose thousands of individuals named in the documents to detention, harm or putting their lives in danger. The site's memo on The Fifth Estate nevertheless quotes a Guardian interview in which Cumberbatch revealed he was initially concerned the film might make Assange a "cartoon baddie". The British actor says during the interview: "I think I may get my head bitten off by Disney for saying so, but everyone agreed with that," though it is clear his comments are about an early stage script.

WikiLeaks says it has multiple versions of the film's screenplay, including a "mature version, obtained at a late stage during the principal photography of 2013" which is the iteration it has posted in full. The Fifth Estate, which also stars Daniel Bruhl, Laura Linney and Stanley Tucci, is released in UK on 11 October, the US a week later and in Australia on 7 November.

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