British police investigated the cases of some 700 servicemen and women who took part in tests at the defence ministry's Porton Down laboratories on Salisbury Plain, western England, between 1939 and 1989.

Many of them believed they had been taking part in experiments to find a cure for the common cold but say they were instead subjected to exposure of mustard gas and hallucinogenic drugs such as LSD.

Ruling out a criminal trial, the Crown Prosecution Service said it had given "thorough and careful consideration to all the evidence submitted in a number of sample cases and decided that it is insufficient to prosecute in those cases.

"Inquiries continue in one further case," the prosecutors' statement added.

The police investigation, codenamed Operation Antler, was launched in September 1999 when officers began probing allegations made by veterans about the tests.

Senior prosecutor Kate Leonard, said: "The ensuing police investigation was extensive and involved statements being taken from many hundreds of veterans. A wide range of inquiries was made relating to the nature of the work carried out at Porton Down."

Ken Earl, chairman of the Porton Down Veterans Support Group, said his members would be very disappointed at the prosecutors' decision.

Earl, 69, said veterans' lives had been blighted by the tests and they typically suffered from heart, nerve and spinal problems and depression.

WAR.WIRE