The Ministry says the new buildings were built to replace testing chambers which had reached the "end of their useful life". Since the Twenties, Porton Down has been exposing soldiers to a variety of chemicals, including nerve gases, mustard gas and the riot control weapons CS and CR gas.
Some of the troops were subjected to tests in the chambers without protective equipment or clothing. It is believed to be the longest-running chemical warfare testing programme on humans in the Western world. There was one brief break in 1951 after a serviceman died in an experiment in which nerve gas was applied to his arm.
At the moment, between 100 to 200 servicemen each year are involved in the tests, which Porton Down says are necessary to evaluate the effects of chemical weapons on unprotected humans and to develop protection against such weapons.
A growing number of servicemen who volunteered for the tests are now campaigning for compensation,believing that their health has been damaged.
Porton chiefs insist that, aside from the one death, the experiments have not harmed the health of any of the servicemen who, it claims, were exposed to "very low and medically safe concentrations" of chemicals.
In letters to Labour MP Ken Livingstone, Porton Down has disclosed that the new complex was built last year to "replace four existing buildings which had reached the end of their useful life and had become too expensive to refurbish".
Porton Down added that these old buildings "consisted of a respirator fitting and testing building, two chambers and a bathhouse/store".
Two of the buildings dated from 1918, while the others were built after the Second World War.
Mr Livingstone said: "This new complex shows that Porton is intent on carrying on testing on humans. Surely the time has come to put an end to these dangerous and harmful experiments."
The complex was built under contract by Alfred McAlpine Ltd, although it was completed 12 months behind schedule. The letters to Mr Livingstone also reveal that, following two inspections, the watchdog body the Health and Safety Executive (HSE) recommended that Porton should tighten up its safety procedures.
In the most recent inspection in 1988, the HSE recommended that "a system of interlocking doors would render the chance of unexpected exposure to chamber operators remote". After the other inspection in 1983, HSE recommended that detailed written codes of practice be drawn up.
Mick Roche, of the Porton Down Volunteers Association, which represents soldiers who were tested, said the MoD ought to hold a proper medical inquiry to determine whether the health of any of the association's members had been harmed. He said the US had stopped testing nerve gas on humans in 1975. "Why has Porton Down decided to still use nerve gas when America bailed out 20 years ago?"