Mr Maddison was 20 when he died

Ronald Maddison, from County Durham, died in 1953 after being exposed to the nerve agent Sarin.

An ex-army ambulance driver called to help Mr Maddison described what he saw when he arrived at the facilities.

Alfred Thornhill, said: "It's 51 years ago and I've never forgotten what I saw. I want to say how sorry I am."

He continued: "He [Mr Maddison] was convulsing and foam was coming out of his mouth.

Alfred Thornhill saw Mr Maddison convulsing

"Then he was taken into the medical centre where there were scientists and medical people.

"They just threw him onto the bed and gave him a big injection.

"It was a terrible atmosphere - they were all panicking. They couldn't handle what they were looking at.

Mr Maddison, a Swindon-based RAF engineer, was one of many human volunteers involved in tests from 1939 to 1989.

He took part in what he thought was an experiment to find a cure for the common cold.

Still to give evidence are serving Porton Down staff and the former Wiltshire detective who led the five-year investigation into what happened.

The original inquest into Mr Maddison's death was held in secret for what were claimed to be "reasons of national security".

Last year Lord Chief Justice Lord Woolf quashed the original verdict of death by misadventure and said a new inquest was needed in the interests of justice.

The inquest is expected to end in early July.