During the early 1960s millions of people were exposed to clouds of radioactive xenon gas from the Harwell Nuclear Research Establishment, the BBC has discovered.
The gas was sprayed from the top of an incinerator stack at the Oxfordshire facility.
Over four lunchtimes in July 1961, men in white body suits released large clouds of the gas over English towns and villages to help estimate what effect a biological or chemical warfare attack would have on Britain.
Until now nobody outside of a select few ever knew what they did, why they did it or what the implications for public health could be.
The details have long been hidden away in document PTP 794, which has never been made public before.
The BBC was tipped off about the existence of the document, which was kept among records at the Met Office.
There is a finite chance, however small, of people suffering from cancer or genetic damage after inhaling this substance
Dr Chris Busby, Low Level Radiation Campaign
It details experiments that were commissioned by senior staff at Porton Down's Microbiological Research Establishment in Wiltshire.
Yet nobody there, including a senior scientist who worked at Porton during that time, seems to remember what went on.
This is despite the fact that the Harwell trials were described as "crucial" in the minutes of a meeting of the Offensive Evaluation Committee held at the War Office.
Graydon Carter, historical consultant for Porton Down, said he had no knowledge of the trials.
He added that he thought this work could have been done by meteorologists, rather than chemical defence scientists.
Perhaps some of those involved might rather forget what they did, given the nature of these secret tests and the fact that not everything went to plan.
The document reveals how when the first plume of radioactive xenon was released, it travelled so fast that scientists waiting to measure it 62 km away did not have time to monitor it before it swept over their heads and on towards London.
A further test was supposed to have taken place another 20 km on, but this never happened due to what is described in the document as "a misunderstanding."
On other occasions, plumes of radioactive xenon were known to have sailed over Reading and Bracknell in Berkshire, and on towards Guildford in Surrey in a 72 km arc from Harwell.
Other tests carried out during this period included spraying the British coastline... with live bacteria bred from a toilet seat
It is now feared that the trials could have led to the death of one scientist and may have damaged the health of local people.
The relatives of Charles Wallington, a meteorologist involved in the Harwell experiments who later died of a rare blood disorder, believe his death may be linked to the work he conducted.
Critics have told Radio 4's Document programme that the trials were totally irresponsible.
Human rights issue
Although the amounts of radioactivity involved were thought to be very small, Dr Chris Busby, a physicist and consultant for the Low Level Radiation Campaign described the tests as "arrogant and bordering on criminal behaviour".
He said it was typical of the "high-handed approach" taken at the time to carrying out such experiments.
"On a human rights basis it is absolutely wrong to invade a person's body with a substance which may cause them harm, without telling them," he said.
He added: "There is a finite chance, however small, of people suffering from cancer or genetic damage after inhaling this substance."
In July 1963 spores of bacteria were thrown out of tube train windows
Other tests carried out during this period included spraying the British coastline from Lyme Regis to Torbay with live bacteria bred from a toilet seat at Porton Down.
The idea was to see how long Ecoli-162 could survive in the atmosphere after being sprayed into the air from a ship called the Icewhale.
There were 43 such trials between 1963 and 1971. Amongst 250 other clandestine trials were several involving the London Underground.
In July 1963 spores of bacteria, housed in the sort of compact cases you would find in most women's handbags of the day, were thrown out of tube train windows.
Scientists measured that by tea time they had travelled up to 19 km across the capital's tube system.
It had previously been thought that only harmless dust was used in these experiments.
Spraying bugs from planes was also much in vogue. Scientists came to realise that an enemy could contaminate vast areas in seconds.
Clouds of bugs or selected chemicals drifted as far as the Scottish borders and into Wales.
British cities were also targeted following the discovery that warm air rising from urban areas affected the way bacteria spread.
This edition of Document was broadcast on 12 September at 2000 BST on BBC Radio 4.