When terrorists targeted the Japanese subway, it was their use of Sarin gas that caught the public fear. When Saddam Hussain's crimes are mentioned, it is invariably using chemical weapons against the Kurds that is cited.

And some of the mystery is attached to the name of Porton Down, the secret chemical weapons centre in Wiltshire.

The centre, made up of forbiding buildings in 7,000 acres near Salisbury, was set up in 1916 at the height of WWl.

Patrick Mercer, a retired army officer, spent several weeks there on courses designed to tell soldiers about chemical warfare.

"It was hideous," he said, "a hutted camp, where it seemed to do nothing but rain. There were a series of bunkers to which you were thrust from time to time to be gassed with CS gas and to go through ghastly exercises underground wearing a gas mask."

Ronald Maddison: Took part in experiments in 1953

During WWII Porton Down started researching a new menace - biological weapons, but during the Cold War chemical weapons became the top priority.

For many years, the mere fact that there was a chemical weapon research centre there was secret, but after it was admitted in the late 1960s, it became the most controversial military establishment in the UK.

To test the effectiveness of nerve agents such as Sarin, servicemen were offered about �2 and a pass for three days' precious leave if they voluteered to take part in tests.

Rob Evans, a journalist researching a book into the experiments, said the main reason people volunteered was because they were bored with life at their own military establishments.

"They wanted to get away for any type of break, just anything. As soon as something came up. . .they would step forward, say yes, I'll take that.

"But sadly very few actually knew what Porton Down was, or what they were letting themselves in for."

Allegations

Wiltshire detectives are investigating allegations that in 1953 one serviceman, Ronald Maddison, died after taking part in a Sarin gas experiment. It is claimed that he thought he was taking part in a programme designed to find a cure for the common cold.

But the Maddison death was not the only thing to go wrong at the centre.

Rob Evans said: "The two most embarrassing accidents, and they are more tragic than embarrassing, were the death of Ronald Maddison and also the death of one of their own scientists Geoffrey Bacon in 1962, who died of plague."