WARNING: THIS BLOG CONTAINS GRAPHIC IMAGES WHICH SOME READERS MAY FIND DISTURBING

The boy is naked from the waist up and lying on the ground. He has five neat little bullet holes in his chest. Beside him lie five men, also all dead, blood spilling from their gunshot wounds.

The camera which is filming this scene drifts unsteadily over the bodies - and then back towards the corpse of the boy.

The boy's name is Balakandran Prabhakaran, and he is the 12-year-old son of the Tamil Tiger leader Velupillai Prabhakaran. It is May 2009 and we are in the final few hours of the brutal 26-year-old civil war between government forces and the secessionist rebels of the Tamil Tigers, (LTTE).

These few moments of video footage - which we will be revealing for the first time in our new film, Sri Lanka's Killing Fields: War Crimes Unpunished, to be broadcast at 10.55 this Wednesday on Channel 4 in the UK - represent more than just a record of a grotesque individual crime. They represent the latest in a mounting catalogue of evidence of war crimes and crimes against humanity; evidence which points - ever more firmly - to the highest levels of the Sri Lankan government.

I'm writing this in Geneva where - behind the scenes of the United Nations Human Rights Council - frantic lobbying is going on over a modest resolution which calls on the Sri Lankan government to implement the proposals of its own Lesson's Learnt and Reconciliation Commission and institute a credible independent inquiry into the allegations of war crimes which should report back to the UN in a year's time.

The vote is significant, partly because it represents a real test of the UN's ability and willingness to confront the issue and its own failure to carry out its 'responsibility to protect' over the appalling carnage at the end of the Sri Lankan civil war.

The Sri Lankan regime, headed by President Mahinda Rajapaka and his brother, the defence secretary Gotabaya Rajapaksa, are doing as they have always done, denying every allegation, claiming that the footage in our films is fake and angrily denouncing UN estimates of up to 40,000 dead as a wild exaggeration.

But they are also mounting a campaign to defeat the resolution - relying to a great extent on claims that it - and indeed all the international calls for truth and justice - are part of some kind of Western agenda to harass the small sovereign state of Sri Lanka.

It is a rather desperate claim, designed to rally support among the non-aligned nations, but it is unlikely to succeed, partly because it is transparently hypocritical. During the last stages of the war the Rajapaksa regime was only too keen to cast its war as a component of the West's Global War on Terror. It constantly invoked the language of the GWOT in a largely successful attempt to buy the acquiescence of the rest of the world while it set about the annihilation of the LTTE and - as the evidence now suggests - the deliberate targeting of Tamil civilians in the north east who were perceived, reasonably accurately, as supporting the LTTE.

Just how cynical the Sri Lankan objection to 'foreign interference' is, was vividly illustrated by a keynote speech that President Rajapaksa made to the UN in 2010 when he effectively warned the world to back off - insisting "imposed external solutions breed resentment and ultimately fail."

"Ours by contrast." he pronounced, "is a home grown process."

Up to a point.

As we reveal in the film on Wednesday night - courtesy of some secret filming by the Bureau of Investigative Journalism - this 'home grown' speech was actually written by the British PR company Bell Pottinger, who had a team in the President's Office.

But the rest of the world does have very real questions to answer about why they failed to stop this appalling tragedy from happening. Sir John Holmes, the then UN Humanitarian Coordinator who visited Sri Lanka on at least two occasions in the last few weeks of the war, was remarkably frank when I interviewed him. This is what he said:

"There was a bit of a diplomatic dance around all this, with everybody knowing that the end of this was going to be an inevitable military victory for the government and the inevitable defeat of the LTTE, and it was a question of waiting for that to happen, hoping it happened as quickly as possible and that it happened with as few civilian casualties as possible."

He then paused for a moment, as though considering what he had just said. Then he added:

"That may sound a bit cynical, but that is the reality of what I was observing."

What happened in Sri Lanka cannot be forgotten. This isn't simply a matter of achieving formal justice.

In the north east of Sri Lanka just now there is a brutal and repressive clamp-down on the Tamil population. Thousands are still homeless while the military has seized land and homes.

Around the world a generation of Tamil youth are burning with anger. If the world does not take action to find the truth and make the perpetrators face justice, that anger may find its expression in a tragic, awful, repetition of history.

Sri Lanka's Killing Fields: War Crimes Unpunished can be soon on Channel 4 at 10:55pm, Wednesday 14 March.

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