DSTL's technical director said last week he welcomed the news that no further action would be taken against the scientists accused of testing deadly chemicals on human guinea pigs at Porton Down.

Speaking after the announcement by the Crown Prosecution Service, Dr Rick Hall said: "I very much welcome the decision that after a comprehensive four-year investigation there is to be no further action.

"I understand the CPS has concluded there is insufficient evidence and that the files submitted by the police failed the first stage of the test."

He said that what had been reported in the press so far was only one side of the story.

"Some reports, although the CPS has said there is no case to answer, have implied there really is something underneath," said Dr Hall.

"A lot of the reports make it look as if it was an enquiry into deaths - there weren't deaths.

"There was one very unfortunate death at Porton Down, and that was Ronald Maddison.

"What has been investigated here is claims by ex-volunteers that they were duped into taking part in tests.

"That is what this is all about and that is what the CPS has dealt with."

Dr Hall said that the ministry of defence was very proud of the volunteer project that took place at Porton Down during the 1950s and 1960s, and very grateful to the servicemen and women who knew what they were taking part in.

He added: "I very much regret the anxiety and distress caused to ex-volunteers and ex-members of staff by this investigation.

"I understand why it had to happen but I do regret the anxiety and distress caused by the issues that happened 50 or 60 years ago.

"Those ex-volunteers who have concerns can ring us, and we will give them full details of what happened to them.

"And for those people who believe their health has been damaged there is a medical assessment programme and we would very much encourage them to put themselves forward for this."

Ex-volunteers can contact Dstl on 01980 613121.