Britain's obsession with Saddam Hussein's still-to-be-discovered weapons of mass destruction is about to be turned on its head following a retired engineer's willingness to testify about what he personally witnessed at the chemical weapons establishment at Porton Down, UK.

Although it is 50 years since Alfred Thornhill witnessed the death of 20-year-old Ronald Maddison in Porton Down's gas chamber, his testimony is expected to ignite a debate about Britain's contribution to the development of chemical weapons -- something Saddam was accused of harbouring.

One of them is the nerve gas Sarin. UK scientists experimenting with the dosage that could prove lethal for human beings, tried it on Maddison and hundreds of other volunteers who believed they were participating in an experiment to find a cure for common cold.

But minutes after Sarin was dripped on to his skin, Maddison started to turn blue. He died 24 hours later after uncontrolled vomiting and repeated bouts of convulsion.

Thornhill, who witnessed the young man's death throes, has now broken his 50-year silence to tell police what he saw and how he was threatened with imprisonment if he ever spoke out.

"I had never seen anyone die before and what that lad went through was absolutely horrific," he says in his testimony.

"It was like he was being electrocuted; his whole body was convulsing. I have seen somebody suffer an epileptic fit, but you've never seen anything like what happened to that lad. The kid was vibrating and there was all this terrible stuff coming out of his mouth… it looked like frogspawn or tapioca.

"I saw his leg rise up from the bed and I saw his skin beginning turning blue. It started from the ankle and started spreading up his leg. It was like watching somebody pouring a blue liquid into a glass…

"Then one of the doctors produced one of the biggest needles I had ever seen. It was the size of a bicycle pump and went down into the lad's body. The sister saw me gawping and told me to get out."

Although British authorities claim such experiments are no longer carried out, Thornhill's memories will fuel speculation about what other chemical weapons British defence scientists have experimented with long before Saddam came to power.

As British coalition troops continue searching for weapons of mass destruction in Iraq, Thornhill's testimony will add a new dimension to the motivation behind that search.