Between 2001 and 2003 Wiltshire police submitted a number of files to the prosecution service on 66 people tested at the Porton Down Ministry of Defence research facility over a 50-year period under what was known as the Human Volunteer Observer Scheme.

There were 11 suspects in total contained in nine relevant advice files. During the course of the investigation two suspects died, leaving nine.

The Crown Prosecution Service announced a decision not to prosecute in July 2003 but promised to reconsider that decision in the light of evidence presented during a 2004 inquest into Leading Aircraftman Ronald Maddison's death.

An inquest jury concluded unanimously in November 2004 that Maddison, 20, from Consett, County Durham, was unlawfully killed in May 1953 when he dropped dead shortly after drops of Sarin nerve gas were placed on his arms at Porton Down.

In a statement published yesterday, senior Crown prosecutor Kate Leonard, who conducted a review of the Porton Down files, said that there would be no criminal charges as a consequence.

"I have decided there is still insufficient evidence available to prosecute any person with a criminal offence over the testing which was carried out," she said.

A verdict of misadventure was initially recorded on Maddison's death at an inquest in 1953. But the findings of Wiltshire police's inquiry, launched in 1999, into Porton Down's human testing programme persuaded Lord Chief Justice Woolf to order the second inquest into his death.

A verdict of unlawful killing was recorded after the inquest heard Maddison had been taking part in what he believed were tests to find a cure for the common cold.

Last month it emerged that his family had been given ?100,000 in compensation by the MoD after it abandoned a high court judicial review application on the second inquest's verdict decision in February.

Ms Leonard went on: "When I made the original decision not to prosecute in July 2003 there was one case outstanding where I was awaiting further evidence from the police. The final information in that case was received in November 2004.

"I decided not to make a decision in that case until I had considered the evidence from the inquest. Having done that, my decision is that there is insufficient evidence in relation to that case as well.

"In reaching these decisions I considered the evidence from the inquest into Mr Maddison's death to see whether it had any impact on my previous decision. I also looked at recent cases, which have been before the courts since 2003, which have clarified the legal issue of consent."

The Crown prosecutor added: "It was considered proper in light of the age and, in some cases, failing health of the potential suspects to reach a decision in 2003 as to whether the evidence then available provided any realistic prospect of conviction despite the [then] forthcoming inquest."