The video was purportedly smuggled out of Sri Lanka by a media freedom group and passed to a British television station.

The grainy mobile phone footage shows a grassy field scattered with the corpses of naked men. Men in Sri Lankan army uniforms lead a naked, bound and blindfolded man to a spot, force him to kneel and kick him in the head. He is then shot once in the back of the head.

Late yesterday the Sri Lankan Government was dismissing the video as a fake but promised there would be a full military investigation if it proved genuine.

Local and international media were prevented from covering the offensive – the coup de grace in a campaign to eliminate the Tamil Tigers – but there have been persistent rumours of executions and extrajudicial killings.

The footage was released by Journalists for Democracy in Sri Lanka, a non-government group that has campaigned for press freedom in the conservative south Asian country in recent years.

Late yesterday a senior spokesman for the Sri Lankan Government, the head of its Peace Secretariat, Dr Palitha Kohona, dismissed the video as a fake.

”Our initial reaction is still to treat this [footage] with the contempt it deserves, but to dispel any iota of doubt, there will be the proper military investigation,” Dr Kohona told the Herald.

”We think it is a fake. There are reasons for saying this. Apparently this incident occurred in January and it took a long while for them to air it, which makes us very suspicious.”

In January the Tamil rebels in the north were still ”on the run” and no prisoners had yet been taken, Dr Kohona said.

Dr Sam Pari, of the Australian Tamil Congress, said the footage was further cause for Australian sanctions against Sri Lanka.