Operation Antler was a 1999 police investigation into experiments conducted on humans at the British government's Porton Down research establishment between 1939 and 1989.
In July 1999, the Wiltshire Constabulary opened an investigation into allegations by a former serviceman of malfeasance at Porton Down Chemical and Biological Research Establishment.
The inquiries established that a number of the participants in the Service Volunteer Programme claimed to have been tricked into taking part in experiments.
The Second World War was the peak period for testing, and much of this concerned mustard gas, with as many as 8,000 volunteers being exposed.
The CPS decision was reviewed following the verdict of unlawful killing at the inquest in November 2004 into the death on 6 May 1953 of volunteer Ronald Maddison.