Now it can be told. Nearly 200 people died and 3,000 were injured in Britain's worst industrial accident, after a series of fires and explosions ripped through one of the country's biggest petrochemical works late in the evening of 23 February.

The scenes of carnage extended far beyond the limits of the industrial complex. Nearly half the dead were football fans fleeing from rioting on the pitch at a Coca-Cola Cup semi-final. As they sought to escape the violence, they choked and died in their cars when dense clouds of toxic gas rolled across the motorway.

Traffic jams trapped thousands of people out in the open as local residents ignored police advice to stay indoors and keep all windows and doors closed. Instead, trying to evacuate themselves and their families, they blocked roads and prevented ambulances and fire-fighters from reaching the fire.

According to the Chief Constable for the county, 650 people suffered serious chemical burns. The entire NHS serious burns capacity collapsed under the strain and a fleet of air ambulances flew the injured to France and Germany for treatment.

Then the power cuts began. Swathes of southern England were blacked out as electricity generating stations along the coast shut down. Oil pollution from a crippled tanker was clogging their sea-water cooling intakes.

The tanker, which had been diverted away from the blazing refinery, collided with a liquid natural gas carrier which exploded and sank with the loss of all 20 crew. But the Liberian-registered tanker, carrying 180,000 tons of crude oil, was holed and started losing its cargo rapidly. One eyewitness described how she saw figures thrown into the water by the fireball of the gas explosion, only to be engulfed by blazing crude oil.

As the morning of Thursday 24 February dawned, the carnage of the previous night was clear to see. In addition to the human casualties, farm livestock in the rural hinterland of the industrial area lay dead in the fields. Thousands of birds were dying as the oil slick stretched along the coast.

Five thousand self-evacuees were being cared for in reception centres. Schools and offices were closed. Businesses unrelated to the petrochemical plant were unable to open. Normal life across a large area had come to a standstill.

While police and ambulance services struggled at the scene of the accident, while every second counted, committees of civil servants in Whitehall spent their time like medieval scholars, debating whether the Department of Employment or the Home Office or the Civil Contingencies Unit of the Cabinet Office should be the 'lead' department in co-ordinating the Government's response. More used to providing considered policy advice than taking prompt action to avoid death and disaster, civil servants pondered every paragraph and nuance in the Home Office pamphlet Dealing with Disaster - a form of displacement activity as chaos and confusion engulfed the real world.

Britain has more than 300 high-risk industrial installations classified as 'top-tier' sites under the terms of the Control of Major Industrial Accident Hazards regulations. These contain toxic or explosive materials in such quantity that a severe accident could injure or kill people living off-site. Strictly controlled by the Health and Safety Executive, they are scattered across the country, sometimes in surprisingly rural areas, from Newmarket in Suffolk to New Abbey in Dumfriesshire. But several are concentrated together near major industrial ports such as Bristol, Southampton, Canvey Island in Essex, and Immingham in south Humberside.

Now that the Cold War has ended and the Government is no longer obsessed by civil defence against nuclear Armageddon, officials and ministers have begun to realise that they should have been worrying about the risk of industrial disasters all along. Not before time.

The disaster described opposite did not, of course, really happen. It was a paper scenario set out before senior officials. In the pleasant surroundings of a country house near York, now used as the Civil Service Emergency Planning College, officials from departments as diverse as Health and Defence rehearsed 'Exercise Green Amanita' - what they would do if we had lost large swathes of England in an industrial catastrophe.

Perhaps the most striking lesson is that there is very little that central government could do. One man from the Ministry of Agriculture, Fisheries and Food, newly appointed to responsibility for MAFF's disaster planning, talked lovingly of the shiny emergency control room that came with the job. At the touch of a computer button he could summon from an electronic database not only detailed maps of the whole country, but also precise information about what crops were grown where. If some accident so contaminated agricultural land that food production had to be stopped, the computer would tell ministers and officials instantly what was being affected.

Other departments, such as Trade and Industry, also have emergency control rooms with secure communications. Much of the job of central government in an emergency would be to co-ordinate the flow of information to the staff of these rooms. But outside Whitehall, the real business of saving lives and hospitalising the injured would be exclusively the concern of the 'blue light' services - police, fire and ambulance. They practise their emergency preparations frequently.

The Nuclear Installations Inspectorate practises a simulated nuclear emergency each year, even though the probability of a serious accident causing even a couple of off-site casualties is less than one in a million per reactor each year. In contrast, in a seminal risk-assessment of the concentration of oil refineries and chemical works at Canvey Island, the Health and Safety Executive calculated that the risk of killing or seriously injuring 1,500 people at a stroke was one in 5,000 a year.

It is thus 200 times more probable that Britain will suffer major loss of life in a non-nuclear than a nuclear accident. But until Green Amanita, the Government had never rehearsed a non-nuclear industrial disaster.

While the accident scenario put before the civil servants may have been fictional, their confused reaction during the York weekend was real. And, to a certain extent, it was deliberate. As one of the authors of the scenario pointed out later: 'We wanted to put the concept of 'lead' government department under extreme pressure.'

Even as the pressure mounted, some officials retained their sang-froid. The civil servants were grouped in operations rooms learning news of disaster succeeding disaster not only from simulated official reports handed out by the exercise controllers, but also from specially devised television broadcasts. As the forest of map marker pins grew thicker around the crisis zone, one senior civil servant, in the best classicist's tradition, was heard to tut: 'This really is piling Ossa on Pelion.'

Part of the ordeal was a simulated press conference with questioning from four journalists who had been invited to represent the outside world at the exercise. The media's irritant value can be judged by the reaction to a final twist in the scenario, when a helicopter full of Fleet Street's finest was supposed to have crashed, killing all on board. Smiles broke out for the first time and a ragged cheer went up from the hard-pressed officials.

Ultimately, Exercise Green Amanita was strangely reassuring, despite the flaws it highlighted. It showed that, prompted by the Health and Safety Executive, the Whitehall machine is taking industrial risk seriously - at the highest level. Once the lessons of this one have been digested, there will be other exercises until the drawbacks are ironed out.

Amanita is a family of toxic toadstools - among them the Death Cap, Destroying Angel, and Fly Agaric. The green variety is fictional and, thanks to that eponymous exercise in Yorkshire, looks set to remain so.

(Photograph omitted)