One by one they held out their left forearms while drops of the deadly nerve agent sarin were applied to a layer of serge and flannel taped to their skin.

Within an hour one of the men, 20-year-old Leading Aircraftman Ronald Maddison, was dead. Yesterday a coroner opened the inquest into his death.

David Masters told the inquest jury that the gap between the death being investigated and the inquest into it was the longest on record. A fresh inquest was ordered after a seven-year campaign by the dead man’s family to establish the truth about his death.

A verdict of misadventure, reached by a coroner at a secret hearing within days of Maddison’s death, was overturned by the Lord Chief Justice 18 months ago.

Mr Masters told the jury that it would be necessary to take them back to the Britain of more than 50 years ago, where television was a novelty and the Second World War had given way to the Cold War. The US had exploded the first hydrogen bomb in the Pacific, the Russian dictator Joseph Stalin had died and the Korean War was ending.

Despite the time gap, the inquest, which is expected to last eight weeks, will hear evidence from 50 witnesses and read statements from 43 more.

Many of those who took part in the sarin trials, including all the scientists involved, are dead. But the jury will hear from a number of witnesses who volunteered for experiments at Porton Down, including two who were in the hyperbaric chamber with Maddison when the fatal dose was administered.

Mr Masters told the hearing at Trowbridge, Wiltshire, that Maddison, originally from Consett in Co Durham, had been stationed at RAF Ballykelly in Northern Ireland when he volunteered to take part in trials at Porton Down.

Sarin and another slightly less lethal nerve agent, tabun, codenamed GB and GA respectively, had been discovered by Nazi scientists. Britain, Canada and the United States had signed a secret tripartite agreement jointly to develop an “offensive capability” with nerve gas.

Scientists at Porton Down, then under the auspices of the Ministry of Supply, were attempting to find out sarin’s LD50 — the lethal dose at which it would kill at least 50 per cent of those to whom it was administered.

Volunteers, mostly recruited among young national servicemen, donned gas masks and sat in the chamber in batches of six while the drops of sarin were applied to their arms.

At 10.17am on May 6, 1951, volunteer 745 Ronald Maddison became the fourth of his group to have 20 10mg drops of sarin applied to his left forearm. At 10.40 Maddison said he felt “pretty queer”. He was let out of the chamber and went for a short walk to a bench where he was examined by another doctor.

He told the doctor he couldn’t hear what he was saying. An ambulance was called, but before it arrived Maddison had fallen unconscious. Shortly after 11am he was dead.

The inquest heard for the first time the report of the original post-mortem examination which revealed that Maddison died because his lungs clogged with viscous mucus. The cause of death was recorded as asphyxia.

Among those in the public gallery yesterday were a dozen elderly men who had taken part in trials at Porton Down. Most claim to have suffered long-term damage to their health because of tests they say were never explained to them.

Maddison’s family is represented at the hearing by his sister, Lillias Craik. Mrs Craik, 70, listened for the first time to the official account of how her brother died.

She said: “My family has never known the truth or why he had to die so young. We are grateful to the coroner for holding this inquest so we can finally know the truth.”

The hearing continues.