A KEY witness in the death of a North-East airman deliberately exposed to nerve agents has broken his 50-year silence.

Ronald Maddison, of Consett, County Durham, died at the Government's top secret chemical weapons laboratory in Porton Down, Wiltshire, after he was used as a human guinea pig in tests with the deadly nerve agent sarin.

On the eve of a fresh inquest into his death, former ambulance driver Alfred Thornhill, 70, has told for the first time of horrific scenes in the RAF airman's last hours.

"What that lad went through was horrendous - it shouldn't have been allowed to happen to anybody," he said.

"We talk about Saddam Hussein gassing his people but what we did at Porton Down was the same ... I want his family to have justice."

Mr Thornhill, who was 19 at the time, was on a month-long posting to Porton Down as part of his National Service.

He answered an emergency call, arriving at Porton Down's gas chamber to see the convulsing body of 20-year-old Mr Maddison "thrashing around" on the floor. He described what looked like "frogspawn or tapioca" coming from the airman's mouth.

At Porton's medical unit, he witnessed further nightmarish images as the airman's leg rose from the bed and the skin turned blue.

Afterwards, he was warned that if he ever told anyone what had happened, he would be sent to prison.

His testimony will form a key element of the reopened inquest into Mr Maddison's death, which is due to be heard in the next few weeks.

An original Ministry of Defence inquest, held in secret in 1953, recorded a verdict of misadventure.

Although a police inquiry into events at Porton Down found insufficient evidence to mount a prosecution, their findings were passed to Lord Chief Justice Woolf, who ruled that the inquest must be reopened. Lawyers for Mr Maddison's family are hoping for a verdict of unlawful killing.

Mr Maddison's sister, Lillias Craik, welcomed the news that Mr Thornhill had come forward.

"It is a turn-up and strengthens our case," she said. "Hopefully, it will shame them into admitting what really happened. Now we can finally get to the truth."