London on Wednesday said "horrific" footage contained within a film documenting the final weeks of Sri Lanka’s civil war constituted "convincing evidence" of war crimes.
The Channel 4 documentary, which aired on 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 the Foreign Office.
"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 Tamil Tiger fighters had also committed war crimes.
Sri Lanka on Wednesday denied it targeted civilians while crushing Tamil Tiger rebels but said action would be taken if any allegations of atrocities were proved to be true.
The Sri Lankan High Commission in London said images shown in the documentary had not been verified as genuine and the disturbing footage could cause hatred among Sri Lankan communities.
However, it added that a local panel known as the Lessons Learnt and Reconciliation Commission (LLRC) was ready to take note of the claims and take remedial legal action.
"If the allegations levelled by Channel 4 or any other party are found to be genuine, the LLRC will take due note of all such cases and remedial measures will be taken by way of legal sanctions," the statement said.
Two UN investigators have confirmed the video as authentic, but the Sri Lankan authorities maintain it is fake, citing a forensic video analyst on its web site as saying that they were extensively edited.
The 50-minute film, "Sri Lanka’s Killing Fields", also included footage of the aftermath of the targeted shelling of civilian hospitals.
What appeared to be the same images were also shown by Al-Jazeera television in November. The channel said it had obtained the images from an unnamed Tamil source who said they had been taken by a member of Sri Lanka’s military.
Burt warned the Sri Lankan government that reconciliation would be impossible unless there was accountability for the alleged crimes.
"Since the end of the conflict the UK has called for an independent, thorough and credible investigation of the allegations that war crimes were committed … and the UK government expects to see progress by the end of the year," he said.
The international community would revisit "all options available" to press Sri Lanka to fulfil its obligations, he added.
"Unless this is done, Sri Lanka will not be able to move on, and the prospects for reconciliation between Sri Lanka’s communities will be curtailed.
"It is of the greatest importance that this does not happen," he added.
The film also examined atrocities carried out by the Tamil Tigers, emphasising that war crimes were committed on both sides.
They included the use of human shields and the aftermath of a suicide bombing in a government centre for the displaced.
"The footage is probably the most horrific the channel has ever shown," Channel 4’s head of news and current affairs, Dorothy Byrne, said.
"We believe this dossier of visual evidence combined with harrowing eyewitness testimony represents prima facie evidence of war crimes and crimes against humanity committed by forces of the government of Sri Lanka."
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