Two eyewitnesses have come forward for the first time to support allegations that the Sri Lankan army executed two Tamil Tiger rebel leaders after they surrendered, carrying a white flag, at the close of the island’s civil war in 2009.

Their accounts cast fresh doubt on the Sri Lankan government’s claim that the rebels were killed by their own supporters and add to a growing body of evidence of war crimes allegedly committed by the Sri Lankan military during the conflict.

Last week, The Independent published photographs of the 12-year-old son of the Tamil Tiger chief eating a snack after being captured by the Sri Lankan army. Shortly afterwards, he was shot five times in the chest, probably at very close range according to a forensics expert.

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The two new witnesses in what has become known as “the white flag incident” can testify they independently saw from different vantage points the Tamil Tiger leaders accepted into the custody of the Sri Lankan military and escorted from the front line, alive.

One witness, who did not want to give his name fearing retribution against his family in Sri Lanka, worked as a bodyguard to the Tiger political leaders. Badly injured in the last month of the war, he surrendered to save his life and says he reluctantly became an informer for the Sri Lankan army.

Now in London, he says he was taken to the front by members of the Sri Lankan military on the morning of 18 May 2009, and positioned behind an earth embankment. His job was to confirm the identity of the Tamil political leaders as they walked towards the army carrying white flags. He says it looked like a well-organised surrender with hundreds of soldiers, including senior officers with bodyguards, present.

The second witness, a government teacher, also now in London, says he was press-ganged into service for the rebels in the last months of the war. Hours before the incident, he says he also surrendered, knowing that the war was over and it was his only chance of survival. After being searched, he says he was held with others in a derelict building close to the front line. From this position he watched several groups of Tamil Tiger leaders and their relatives walk out of the war zone towards the Sri Lankan army, carrying white flags.

He admits he was surprised to see the leader of the Tiger political wing, Nadesan, his Sinhalese wife and the head of the Tiger Peace Secretariat, Pulidevan, in the first group. Officially the Tigers did not hold with surrender, issuing all recruits with cyanide capsules to wear around their necks to use in case of capture.

Both witnesses say Sri Lankan soldiers went out to greet several groups of surrendering rebels and escorted them over a bridge across a lagoon to waiting vehicles on the other side.

The former bodyguard waited for more than an hour until the military put him in the back of a pick-up truck and drove him away. Along the road he spotted soldiers taking photos on their mobile phones of corpses lying on the ground. As they went past, he recognised Pulidevan and Nadesan’s bodies. Photographs have since appeared on websites abroad showing the two Tiger leaders’ half-naked corpses, with bullet wounds and burn marks on their chests.

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The “white flag” incident also involved approximately 40 other rebels believed to have negotiated a surrender with the Sri Lankan government. None of them have been seen since.

Tamil Tiger rebels conducted a vicious campaign of terror for a separate homeland for decades, using suicide bombers to kill civilians and scores of politicians, including a President and an Indian Prime Minister.

Pulidevan and Nadesan were two of the most senior rebels to surrender. In the hours beforehand, they used satellite phones to send messages to diplomats, journalists and peace mediators to ensure top Sri Lankan officials, including the country’s President, knew of their intention to lay down their arms. They had been assured that their surrender would be accepted if they raised a white flag.

However, the Sri Lankan defence ministry’s website lists Pulidevan and Nadesan as killed by troops of the 58th Division on 18 May 2009.

Sri Lanka’s government holds that the Tiger leaders were shot in the back by their own people, and say the army never executed any individuals.

Allegations the Sri Lankan army and rebels committed war crimes at the end of a brutal war are widespread. According to United Nations investigators, the last months saw repeated and deliberate government shelling of hospitals, food queues and safe zones for civilians.