Authorities in the country have discovered mass burial sites containing thousands of bodies, including scores of children.

Trawls of records including declassified files in Washington have uncovered evidence of the massacres of at least 100,000 people suspected of having sympathy with the North Koreans.

In some cases, American forces are alleged to have been present and in at least one case an American officer authorised a massacre of prisoners believed to have left-wing sympathies.

The Korean War, in which South Korean, American, British and other allied forces fought the North and their Chinese allies to a standstill, was particularly bloody.

It was always known that atrocities were committed by both sides as the battlefront swayed back and forth from the deep south to near the Chinese border.

But under the South’s military dictatorship, the crimes of its own forces were rarely discussed. The new findings suggest there was a pattern of disposing of those suspected of left-wing sympathies as the North Koreans advanced, and then again of those who were accused of collaborating as they retreated.

The investigation, run by a national Truth and Reconciliation Commission, has become deeply controversial, with some fearing it is intended to discredit the right and pro-American attitudes in South Korean politics.

But its work has been welcomed by survivors, such as Kim Jong-chol, 71, who escaped as a 14-year-old from one mass shooting. His father, seven-year-old sister, grandparents, and cousins were all killed.

“Those who witnessed the killings said it was pitiful. Babies were killed with their mothers holding them,” he said. “Now I want the government to find their bodies, and to erect a monument in their memory.”

Mr Kim’s father was a South Korean guard recruited into a local militia after the North’s forces overran the border at the start of the war in 1950.

When the South Koreans and Americans army swept back north, a local police chief in their district, Namyang Ju, ordered those suspected of collaborating to be rounded up, along with their families.

“We were taken to a village storage room,” said Mr Kim. “But I managed to slip the ties on my wrists and run away.”Two days later I found the pit where they had shot the captives. I dug with my hands, and found the bodies of my grandmother and grandfather. I never found my father or sister."
The massacre in Namyang Ju was eventually brought to a halt, but not before 460 had died – one of many such killings documented in painful detail by the Commission.

Research in US archives has found one exchange in which a US colonel gives approval to a massacre in which 3,500 suspected leftists were shot.

Professor Kim Dong-choon, one of 15 commissioners, said there were at least four documented cases of US forces bombing civilians. The Americans feared infiltrators might be among them. In addition, American forces were present at two major massacres of thousands of people, near Busan and at Daejeon.

“For people who are in their 70s and went through the war and the dictatorship this may not all be new – it was like a public secret for those years,” he said. “But for the younger generation a lot of this was unknown, and it is coming as a shock to them.”

British troops are among the only groups to escape serious criticism from the Commission, except for accusations they knew of the killings but did not bring enough pressure to bear on the Americans to stop them as senior partners in the alliance.

In fact, British officers are recorded to have complained to American generals, and at least in one case recorded by the Commission intervened militarily to stop a mass shooting north of Seoul.

In late 1950, about the same time Kim Jong-chol’s family were being murdered, 37 troops from the Royal Northumberland Fusiliers, recently arrived, were dispatched by Brigadier Tom Brodie to respond to reports of killings of civilians.

“They were already being shot,” Private David Strachan, one of two of the group still alive, told The Daily Telegraph from his retirement home in Spain. “It was local police doing it. Our captain approached them, but a revolver was pulled on him.”He gave the order to us to fix bayonets – and then they put their weapons down. I reckon we saved about 400 lives there.“Mr Strachan said after he returned home he saw no discussion of what he had seen.”They were killing refugees all over the place. We witnessed loads of massacres,“he said.”When I got back I never saw anything about it. It was all hushed up."

Richard Spencer in Seoul