The First World War saw the first deployment of chemical warfare when the German army unleashed gas on the British trenches in 1915. The need to develop countermeasures as well as a chemical weapons capability spurred the urgent development of the Royal Engineers Experimental Station on what is now Porton Down. During the war, the facility was at the forefront of British research into phosgene gas, mustard gas and chlorine but was largely mothballed after the end of the War.
By 1920, however, the British had decided that permanent research into chemical warfare would need to be carried out and Porton Down began to accrue permanent buildings, labs and hundreds of scientific and military personnel.
During the Second World War, the site was used to investigate further chemical agents, as well as early experiments with biological elements and their potential for weaponisation - including trials with weaponised anthrax held on Gruinard Island under the innocuous name of 'Operation Vegetarian'.
WWII revealed that the Germans were far in advance of the Allies in terms of chemical and biological warfare and so, despite Britain being a signatory of the Geneva Convention, German technology was explored further at Porton Down.
Officially, research was carried out purely into countermeasures against chemical warfare, but it it known that tests were carried out on human subjects. VX nerve gas was developed at Porton Down in 1952, and one serviceman - Ronald Maddison - died after exposure to Sarin nerve gas in 1953. Sarin was created by German chemists during World War II and was the most potent of the chemical weapons engineered by the Nazis. Like so much of their technology, the Allies co-opted it into their own arsenals and research,
This gas would later be used by Japanese 'death cult' Aum Shinrikyo in their attack on the Tokyo subway in 1995, which killed 13 people and left thousands in various states of injury. An inquest in 2005 found that Maddison had been unlawfully killed by the M.O.D. and his surviving family were awarded compensation.
This case is presented as an individual, tragic outlier by official sources, but it seems fairly clear that testing of chemical and biological agents on human and animal subjects was - and possibly still is - being carried out on an industrial scale by the facility. 20,000 'volunteers' were recruited from the armed forces in a program beginning in the mid 60s. Most believed they were taking part in a trial to help find a treatment for the common cold, and were grateful to get paid expenses and receive additional leave.
In fact, many of them were used in nerve gas experiments and more besides. For example, it is claimed that at least 40 servicemen were deliberately injected with Kyasanur Forest Monkey disease - which has a normal fatality rate of some 28% among humans.
More sinisterly still, it is alleged that scientists from Porton Down were involved with injecting 33 terminal leukaemia patients at St. Thomas' Hospital with the same disease. Catholic Monsignor John Barry raised the issue with ministers in the 1970s, but no formal investigation happened until the early 21st century. Among those tested upon were people suffering from dementia who were legally incapable of consenting to the trials.
3 years after the trials, Kyansaur Forest Monkey disease was formally recognised as a biological warfare agent.
Others who took part in the trials claim to have been unknowingly exposed to nerve gas and raditation and given drugs such as LSD. In 2006, the M.O.D. made payments to four ex-servicemen who claimed to have been unknowingly administered LSD during trials in the 1950s.
Such was the concern over cases of human testing and biological warfare experiments that in 1999 Wiltshire Police opened an investigation into potential malfeasance at the base. 25 specific instances were investigated but only 8 were finally filed with the CPS for prosecution. During the investigation, it was alleged that testing of nerve agents such as sarin gas had been carried out at the base as late as 1983, during testing of Atropine - a potential antidote.
No prosecutions were pursued and the file itself is unavailable on the internet - although a copy is held by the National Archives and marked as 'closed and retained by the Ministry of Defence.'
It has been claimed on the fringes of the UFOlogy community that a UFO that is alleged to have crashed at Berwyn Mountain in 1974 was taken to Porton Down for study.
BBC News Report
BBC News' coverage of the announcement into the first enquiry into experiments carried out at Porton Down