Ofcom said images featured in the documentary, broadcast in June, "whilst brutal and shocking", did not exceed what the Channel 4 audience would have expected, given the pre-transmission warning about the nature of the content and the programme's scheduling at 11.05pm, well after the 9pm watershed.
The media regulator received 118 complaints about the documentary, about issues including impartiality, offensiveness and the broadcast of misleading material, but concluded it had not breached the broadcasting code on any of these counts.
Sri Lanka's Killing Fields, which focused on a UN investigation into alleged war crimes during the final weeks of the country's civil war, included a number of images of murdered and tortured bodies, and also of partially clothed women who, it was suggested in the documentary, had been sexually abused prior to their death.
The documentary featured mobile phone footage, photographs and eyewitness accounts gathered by programme-maker ITN Productions.
The regulator said: "Channel 4 has a unique public service remit to provide programming that is challenging, diverse and likely to provoke debate. Consequently, the broadcaster has a history of broadcasting very challenging material from war zones (including graphic footage) and seeking out the voices and views of those who may not be represented.
"The images included in this programme, whilst brutal and shocking, would not have exceeded the expectations of the audience for this Channel 4 documentary scheduled well after the watershed with very clear warnings about the nature of the content."
On the question of impartiality, Ofcom noted that Channel 4 had put all the significant allegations included in the documentary to the Sri Lankan government and broadcast the limited statement that was provided.
The documentary also included previous Sri Lankan government statements relating to the final stages of the civil war against the Tamil Tigers, including a clip of an official claiming that the first video of an alleged execution shown in the programme was a fake.
Ofcom also said the documentary was only required to maintain due impartiality on its specific subject – the government offensive against the Tamil Tigers in the final stages of the war – and not the conflict as a whole.
"Ofcom therefore concluded that overall Channel 4 preserved due impartiality in its examination of the Sri Lankan government's actions and policies during its offensive and there was no breach of [the broadcasting code]," Ofcom concluded.
In response to complaints that the programme was misleading, Ofcom said Channel 4 had taken reasonable steps to establish that the material included in Sri Lanka's Killing Fields was not faked or manipulated, and had not materially misled viewers in the way it was presented on air.
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Last week Dorothy Byrne, the Channel 4 head of news and current affairs, told the Lords communications committee that programmes such as Sri Lanka's Killing Fields faced PR pressure from the Sri Lankan government.
She said a demonstration held outside the Channel 4 headquarters in London was organised by the Sri Lankan ministry of defence.
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