The revelation, which follows an in-depth study into bacterial releases trials in the Sixties and Seventies, has prompted calls from MPs for an inquiry into the justification for and safety of the tests.
The trials involve releasing bacteria into the air on MoD land to see if army testing equipment can trace the germs.
Such germs, whose behaviour mimics biological-warfare agents such as anthrax, have been shown in a recent independent inquiry to cause pneumonia, blood poisoning and lung infections if breathed in by "vulnerable people". MPs and academics studying germ warfare had believed that all such testing outside secure laboratories had been discontinued because of their controversial nature and because Britain has no offensive biological-warfare programme.
The trials, which have been conducted since the Eighties, are classified and it is not known how many bacteria are being released, and how close the test sites are to residential areas.
Earlier this month, MPs were outraged by the contents of a report by Professor Brian Spratt, a leading Oxford University scientist, into MoD germ-warfare trials in the Sixties and Seventies. His report detailed how bacteria were dropped from aeroplanes over populated areas and from canisters on the London Underground. The revelation of the existence of new trials has prompted calls for an MoD investigation into their safety.
"These tests going on now are dubious at best. They clearly have implications for the environment and human health," said Norman Baker, the Liberal Democrat environment spokesman. "The MoD needs to come clean with a justification for these tests and it needs to be pretty good."
The tests are believed to involve Bacillus globigii, which reportedly can cause disease in humans, and a strain of E. coli.
For people with breathing problems or poor immune systems this E. coli strain can cause septicaemia, fever, pneumonia and chest infections.
"I think these tests are going on all the time. The whole point of these experiments are for devising new methods of detection," said Professor Spratt, of the Wellcome Trust Centre for the Epidemiology of Infectious Disease, Oxford.
The MoD has thousands of acres of land throughout Britain. But most of the testing is thought to take place at Porton Down, the secret MoD research centre outside Salisbury.