The inquiry into Porton Down, the defence research establishment in Wiltshire, has been sparked by an investigation which began two months ago into the death of airman Ronald Maddison in 1953.

Relatives of the 25 dead men contacted detectives following the publicity surrounding the Maddison case.

The former RAF man, then 20, allegedly died after 200mg of deadly nerve agent Sarin B was dripped onto a piece of uniform stuck to his arm.

MoD under scrutiny

Police are expected to interview some 300 surviving ex-servicemen who claim their health has suffered after taking part in similar experiments with deadly nerve agents in the 1950s.

Ronald Maddison died after allegedly testing Sarin nerve gas in 1953

The investigation is hugely sensitive because it could lead to the prosecution of former Ministry of Defence staff and compensation claims by victims' families.

Alan Care, solicitor for relatives of the Porton Down victims, said the "guinea pigs" were tricked into believing they were taking part in research into the "common cold".

But he said proving what was used on the men might be difficult

'I was paid two shillings'

He said: "It has already been admitted that record keeping in Porton Down at the time was not all that it should be.

Gordon Bell, 61, says he was paid two shillings to be a guinea pig

"That is surprising when you consider that they were dealing with some of the deadliest substances in the world."

The go-ahead for the investigation into Mr Maddison's death was given following a long campaign by another alleged guinea pig, 61-year-old Gordon Bell.

Mr Bell, who now lives in Canada, told BBC Radio Four's Today programme that Wiltshire police had arranged to interview him again on Monday.

Gordon Bell on BBC Radio Four's Today programme: "It is alarming, but I'm not shocked"

He said reports of a far wider investigation were "appalling, but I am not shocked".

"I have always felt that the Maddison case was just the tip of the iceberg."

Earlier, he told the Daily Mail newspaper that he was paid just two shillings for the tests he underwent.

'A dirty trick'

"In one test we had to stand in front of a stream of gas which I could not stand for more than a minute," he said.

"My face was stinging, my throat was red raw and my lungs were burning. I was paid two shillings. It was a dirty trick, plain and simple."

Wiltshire police said in a statement: "Wiltshire police have no further comments to make in relation to the story printed in the Daily Mail on 18 October.