MINISTRY of Defence scientists accused of testing deadly chemicals on human guinea pigs at Porton Down are not to be prosecuted, it was announced on Tuesday.

The Crown Prosecution Service said there was insufficient evidence to bring the scientists, who allegedly exposed servicemen and women to toxic nerve gases in the 1950s and 1960s, before the courts.

They said they had given thorough and careful consideration to all the evidence submitted in a number of cases but that there was not enough to charge anyone.

Kate Leonard, senior crown prosecutor, said: "The ensuing police investigation was extensive and involved statements being taken from many hundreds of veterans.

"A wide range of enquiries was made relating to the nature of the work carried out at Porton Down."

Wiltshire Constabulary started a multi-million-pound investigation, code-named Operation Antler, into the top-secret trials of chemical and biological weapons at Porton Down in 1999.

The four-year probe uncovered a catalogue of claims from former military personnel and their families that those who took part in the 'volunteer trials' were deliberately exposed to lethal nerve agents.

Consequently, the decision not to prosecute comes as a huge blow to the many veterans who claim they were duped into volunteering for the tests.

Former servicemen and test survivor Eric Hatherall (60), of Highworth, near Swindon, said: "I feel very sad and very angry that this has happened.

"It's just a cover-up.

"The tests happened - there are no two ways about it, it's just unfortunate they haven't got enough evidence."

He added: "It seems we'll never know now, and that's the sad thing, because if it had come to court, it could have been cut and dried once and for all."

Operation Antler will continue to assist Wiltshire coroner David Masters concerning the death of 20-year-old RAF mechanic Ronald Maddison, which is scheduled to commence in September.

Mr Maddison died in 1953 after scientists placed drops of the deadly chemical Sarin on his skin.

The original inquest, conducted in secret, ruled that he had choked to death in a 'misadventure'.