BBC North East and Cumbria says it had access to secret files for the programme.

Toxicologist Alastair Hay of Leeds University said the briefing notes scientists made suggested they knew the risks they were taking.

"It is quite clear from the result that there was a significant number of individuals who were affected...to have really alerted Porton that they were teetering on the edge of maybe near a fatal dose for individuals," said Mr Hay.

"They were playing with fire, they were exposing people to concentrations which in the event only killed one man but weren't far off perhaps killing a number of others."

Ronald Maddison died after allegedly testing Sarin nerve gas in 1953

A police inquiry was launched in August into the death of airman Ronald Maddison, 20, from Consett, Co Durham, in 1953, after allegations that servicemen were tricked into taking part in lethal tests with nerve gas.

It is claimed that 200mg of sarin, a deadly nerve agent, was dripped on to the arm of Mr Maddison's uniform to test its protective properties.

Since August there have been claims that Wiltshire police are investigating the deaths of 25 national servicemen who took part in tests at the research centre in the 50s and 60s.

Some servicemen, who were given extra pay and time off in exchange for taking part, said they were told the experiments were for a cure for the common cold.

But the Ministry of Defence has repeatedly denied that servicemen were misled in any way.

'Slumped over'

The BBC film features Mike Cox, of Southampton, now 68, who was with Ronald Maddison in a gas chamber on the day he died.

Mr Cox said he was one of six servicemen, including Mr Maddison, who were given drops of a substance unknown to them.

"The chamber itself was temperature controlled and the conditions were unpleasant," said Mr Cox.

"Part of the way through the test Maddison, that was sat there next to me, slumped over. Two people came and sort of half carried him out of the door."

The BBC programme also features relatives of Mr Maddison speaking for the first time about the events.

The Defence Evaluation and Research Agency, which oversees Porton Down, could not comment on specific claims made in the BBC documentary.

A spokeswoman said: "At the moment all we can say is that we are co-operating fully with the police investigation into Mr Maddison's death at Porton Down."