© Warner Bros
Screenshot from the movie V for Vendetta where the British government released a virus that killed thousands of its own people, thereby enabling it to enact strict quarantines and ultimately force totalitarianism on the population. Paranoid fantasy? Apparently not.
In the 50s and 60s, British government scientists carried out secret trials to find out the likely effects of a biological attack. The results, some never published before, reveal how shockingly vulnerable the capital is to an anthrax attack.

It was lunchtime on London's Northern line. Deep underground, passengers were getting on and off the tube trains as normal. Two men boarded a train at Colliers Wood in south London. As the train gathered speed towards its next stop, Tooting Broadway, one of them got up from his seat and dropped a small carton of face powder out of the window. He could have been idly throwing away litter.

As the train sped on, the carton hit the tracks and burst. Out spewed millions of tiny spores, which began to spread throughout the dark tunnels. Dust swabs taken after three days and two weeks showed that the spores had spread as far up the line as Camden Town station in north London, 10 miles away.

This really happened. But the two men weren't terrorists but government scientists. And the spores weren't anthrax spores, but a harmless micro-organism designed to mimic clandestine sabotage with anthrax. This was an official experiment in 1963, and it showed how easily saboteurs could inflict a potentially devastating attack on Britain's capital.

If there had been real anthrax in the carton, many thousands of passengers would have started to inhale the spores, which would have lain undiscovered for some time. The hardy spores can survive for months. Once inhaled, the spores germinate, producing living anthrax bacteria that multiply rapidly. But it can take up to two months for symptoms to appear.

They would then have suffered fever, headaches, chills, chest pains and other symptoms. Some would have appeared to recover, but then almost all would have died abruptly, after quickly developing lung and brain damage and internal bleeding. London would have been gripped by chaos and panic as ministers tried to find out what had happened.

Documents obtained by the Guardian show how scientists secretly conducted a series of trials in the 1950s and 1960s to assess how easily saboteurs could spread deadly germs - and terror - among Britons. Their conclusions make disturbing reading. The trials clearly demonstrated that Britain was highly vulnerable to such a covert strike. "The potential for clandestine biological warfare attack is considerable," the scientists wrote. For many people, that warning now carries more weight than ever; fears of bio-terrorism have grown markedly since September 11, with Americans especially jittery.

Yesterday, the US government launched a criminal investigation into the outbreak of anthrax in a newspaper office in Florida after it was detected in a third person. The bacteria was found in the nose of an unnamed 35-year-old woman. The US attorney general, John Ashcroft, cautioned that there was still no evidence that the cases were caused by terrorists.

Comment: Indeed, it turned out the anthrax came from the US Army's bioweapons research facility at Fort Detrick in Maryland and its release was timed to inflict maximum terror and force through the PATRIOT Act in the aftermath of 911.

Britain has secret contingency plans to cope with a deliberate release of chemical or biological weapons in populated areas. But whatever the preparations, it appears that there is little that the government could do to prevent citizens being infected, or dying, if terrorists do manage to strike with deadly germs.

A document leaked this week to Channel 4 News spelt out that "although the threat of such action is low, the consequences are potentially enormous. It is likely that the number of casualties would far exceed that resulting from any previous major incident in this country."

One of the trials showed how anthrax could be used to rip the heart out of the British government. Around 100ft underneath central London is a huge network of tunnels. They are connected to war rooms and "citadels" to where, in a crisis, ministers and mandarins can retreat and continue running the country.

In mock attacks on these tunnels in 1955, the scientists released non-lethal organisms that simulated the behaviour of anthrax. The first trial showed that the spores "readily penetrated" huge doors leading up to the surface. The second resulted in "heavy contamination" of three government departments, including the building that is now the Treasury.

In the next trial, the fake anthrax was sprayed along the section of tunnels known as "Q Whitehall", which runs below major government buildings from Parliament to Trafalgar Square. The experiment revealed "extensive contamination of many Whitehall buildings". Although the account of the trial is coy, it is probable that the spores would have infiltrated the prime minister's residence at 10 Downing Street.

Clearly, such a strike could disrupt the upper echelons of the government, wreaking havoc and triggering widespread confusion. Some of the tunnels can be entered easily, but a saboteur would probably need the help of a civil servant or government contractor to get into the ones under Whitehall.

The menace of biological weapons was first taken seriously by the British government in the 1930s. For many years, military planners saw germ weapons as part of conventional warfare: "bugs" to be loaded into bombs and fired off at the enemy in another country.

In the second world war, Britain produced 5m cattle cakes filled with anthrax, to be dropped over Germany in retaliation if the Nazis resorted to germ warfare. The idea was that cattle would eat the cakes and be killed, weakening German agriculture. There were also plans to drop anthrax on six German cities.

The prototype weapons had been tested on the now infamous "death island" of Gruinard off the north-west coast of Scotland in 1942 and 1943. On this isolated, uninhabited island, scientists exploded bombs containing anthrax and then studied how many sheep became infected.

A declassified account of these trials recorded that these experiments "demonstrated to the UK and its allies that biological warfare was not only feasible, but practicable and potent". So powerful was the contamination of Gruinard that no humans or animals were allowed to set foot on the island for more than 40 years.

Although germ warfare was not used in the second world war, its threat was present throughout the cold war. In the early 1950s, British officials began to worry that the Soviet Union might instigate biological sabotage attacks as part of an overall assault on the country. Because of these fears, scientists from the chemical and biological warfare establishment at Porton Down, Wiltshire, began the trials, using the simulant organisms to make the experiments as realistic as possible.

Over 18 months, the scientists regularly sprayed organisms around the British Museum's vast underground warehouse at Westwood Quarry, near Trowbridge, Wiltshire. They were trying to discover how a bacterial cloud would circulate inside buildings if a saboteur attacked key government departments in this way. They found that "it is not difficult to predict the course of events should a bacterial spray be released by a saboteur inside a large building... diffusion would be rapid and complete throughout the building".

The scientists also turned their attention to trains, since "all types of transport are now generally recognised as being likely to be one of the most important targets for special operations in a war of the future. It would be particularly important in the early stages to hamper the deployment of troops; at later stages, subversive attacks on trains crowded with both military and civilian personnel might well cause considerable dislocation." Although their report envisages a type of conventional war between recognisable states, it underlines the danger that we may now face from a terrorist attack.

In 1953 and 1954, trials were carried out on a rail line between Exeter and Salisbury. In one set of experiments, clouds of organisms were sprayed within a tunnel, enveloping the train as it rushed through. On other occasions, the organisms were released within trains. Like the London underground experiments in 1963 and 1964, these trials revealed that there would have been many victims if real biological weapons, and not simulants, had been dispersed. Furthermore, the scientists were stumped about how to counteract a sabotage attack.

More experiments were planned, but none was actually done. From these trials alone, it was clear enough that biological sabotage would be highly effective if properly executed. But there was also another reason for the ending of the trials - the government was acutely aware that there would be an uproar if the public found out about covert military tests in which micro-organisms were sprayed around populated areas.

During the cold war, Porton Down scientists conducted more than 200 trials over vast swaths of Britain, disseminating organisms, and later a chemical, from planes and ships. In most of these experiments, the intention was to find out how a biological attack by the Russians would ravage Britain.

But all these trials were wrapped in great secrecy, and the government has started to reveal details only in the past five years. The experiments in the London underground system were deliberately hidden under the innocent title of "ventilation trials". Of these, one senior official wrote: "I am convinced of the vital need for these trials, which impose no hazard to the public, although clearly knowledge of them by unauthorised persons could be politically embarrassing."

Mike Hood, a former Porton Down scientist, says: "If the Ministry of Defence had informed the public, all the trials would not have been worth doing. The public is so ill-informed, and, of course, you would let a potential enemy know what you were doing."

The possibility of biological warfare trials in public places has been off the agenda since the 1970s, as governments have been aware that a sceptical public would not accept such experiments. Curiously, this opposition may fade away as worries about biological attacks intensify.

Porton Down's experiments showed that biological attacks are feasible, but experts continually stress that it would be much more difficult to mount a real attack. There are many obstacles to overcome, such as producing the germs in sufficient quantities, storing them safely, and delivering them efficiently. In the long run, these difficulties may turn out to be the main reason why we don't suffer a biological assault on Britain.