Julian Assange threatened legal action against a film festival in an attempt to pressure them not to show a documentary on the history of WikiLeaks.

Assange, the WikiLeaks founder, threatened to sue the South by Southwest (SXSW) festival in the United States if they broadcast the documentary, WikiLeaks: Secrets and Lies, earlier this year.

The legal threats came to light after media regulator Ofcom rejected a detailed complaint from Assange about the programme on Monday.

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Assange had complained that the programme, which first aired on More4 in the UK on 29 November 2011, was libellous, unfair and had invaded his privacy.

The Australian whistleblower, who is fighting extradition to Sweden where he faces sexual assault allegations, attempted to get the programme banned from public screenings in the US, sending a tersely-worded email, seen by the Guardian, headed “LETTER BEFORE ACTION” to festival organisers. Assange is understood to have sent a similar email to US cable news broadcaster CNBC.

In the email, which carried Assange’s signature, he told a SXSW organiser: “Please also send me full details of SXSW’s formal complaints procedure. This latter request is made without prejudice to any subsequent legal action I may take against SXSW for the screening of this libellous programme.”

He claimed in the email that Oxford Film & Television, the independent British production firm behind the programme, was “under investigation by the UK statutory regulator Ofcom for multiple breaches of the Broadcasting Code”.

Ofcom had undertaken an investigation into whether Assange’s complaints about the programme were justified and ruled on Monday they were not.

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SXSW aired the programme as planned on 9 March and CNBC showed it in the US on 1 March.

Patrick Forbes, the head of documentaries at Oxford Film & Television, welcomed the Ofcom ruling and praised SXSW and CNBC for not caving in to legal pressure from Assange.

Forbes said the threat of a lawsuit meant his company had to pay for costly legal insurance, and that Assange’s threats may have hampered the programme’s chances of attracting a US cinema deal.

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“Julian Assange attacked this film and accused us of being unfair to him. I am delighted that Ofcom has rejected his detailed complaints about our methods and entirely vindicated the programme and its making,” Forbes told the Guardian.

“The film provides the definitive account of a turning point in history. As is the way with such moments, passions run high. But we have striven to make it as a accurate and fair a film to everyone involved as is possible. And I am very glad that Ofcom has recognised that.”

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Assange responded to the Ofcom ruling in a lengthy statement on the WikiLeaks website late on Monday. He said the ruling “glosses over many substantive points of fact” and accused the programme-makers of a producing a “biased and one-sided smear documentary”.

A spokesman for Assange declined to comment further. WikiLeaks had not responded to a request for comment at the time of publication.

Assange is currently taking refuge in the Ecuadorean embassy in London to avoid extradition to Sweden. His most recent public appearance, on the balcony of the embassy in Knightsbridge, included a lengthy attack on critics of WikiLeaks, whom he described as a threat to freedom of expression.

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He wrote to the Leveson inquiry into press standards in April, claiming he had “suffered extensive libels” comparable to Gerry and Kate McCann, who received significant damages from a number of national newspapers over coverage of the search for their missing daughter Madeleine.

He had previously had a complaint not upheld by the Press Complaints Commission about 45 articles, in publications including the Guardian and the Independent, which he claimed were inaccurate and unfair.

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