The Channel 4 documentary, which aired on British television on Tuesday, contained footage of what it said were prisoner executions.
It also showed the bodies of female Tamil fighters who appeared to have been sexually assaulted by government forces.
"I was shocked by the horrific scenes," said Alistair Burt, a minister from Britain's Foreign Office.
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"The recent UN Panel of Experts' report, this documentary and previously authenticated Channel 4 footage, constitutes convincing evidence of violations of international humanitarian and human rights law.
"The whole of the international community will expect the Sri Lankans to give a serious and full response to this evidence," he warned in the statement.
The film included images captured by mobile phone, official army and Tamil footage, satellite imagery and stills shot in the Sri Lankan army's battle against the Liberation Tigers of Tamil Eelam (LTTE) separatists in 2009.
The programme also provided evidence to suggest that the