Ministers are preparing for a flu pandemic that they believe poses a far greater risk to Britons than a terrorist threat.
Government statisticians have warned that a worldwide outbreak of flu is overdue, leading ministers to consider contingency plans for inflatable mortuaries, quarantine facilities, and the evacuation of big cities.
The arrangements follow the outbreak of a bird flu virus in south-east Asia, which has killed 32 of the 45 humans it has infected. Test results last week confirmed that two Vietnamese brothers, one of whom has died, were infected, prompting concerns that the virus can spread between humans. The World Health Organisation has warned of a potential pandemic that could cause seven million deaths. Ministers have been told there is a greater probability of this spreading to Britain than a September 11-style attack.
One senior government source said: "People think terrorist attacks are the most serious threat to us but influenza is currently regarded as the most likely. Our statisticians say an epidemic is overdue. Some of the details are graphic. They're the things that keep me awake at night."
The strain of avian flu circulating in south-east Asia is a particular concern to health officials. Their biggest worry is that it might infect someone who is also suffering from the human form of flu. Because the virus is so good at swapping genes, it could easily pick up characteristics of human flu, making it far more infectious - conditions that health officials say could lead to a pandemic.
The government would aim to prevent a pandemic reaching Britain through screening centres at south-east Asian airports. The idea was discussed in China during the Sars outbreak, two years ago, which infected over 8,500 people in at least 29 countries. "We started this with Sars in 2003," the government source said. "The real plan is to prevent this getting into the country through border controls. That's absolutely critical."
If infected passengers manage to get through screening, the government would want to quarantine them in secure accommodation with residential facilities.
In the worst-case scenario, ministers are preparing for mass evacuations of healthy people from cities. "This is absolutely the last resort," the source said.
Any centre with a large Chinese population - such as Liverpool, Manchester, Birmingham, Leeds, Glasgow and Newcastle - is believed to be particularly vulnerable to a flu outbreak: "It could be any big city with a significant population of people from China if someone were to arrive who had the disease or came into contact with it and went to live with friends or family in one of these cities."
The evacuation plan is understood to mirror a blueprint to evacuate affected parts of London in the event of a terrorist attack. Working on the assumption that roads would be gridlocked by panicking people, the government would coordinate a mass evacuation by rail. Officials are also procuring an inflatable mortuary which can house several hundred dead.
Meanwhile, the Department of Health is assessing how many antivirals it should stockpile. The new class of drug works by blocking the action of a key flu protein and stops sufferers passing the virus on.
Research from the Harvard School of Public Health indicates that giving antivirals to between 50% and 75% of the population would check a virus along the lines of the 1918-19 outbreak, when 280,000 died in the UK and 40 million worldwide.
But, while the NHS is understood to have enough doses for groups vulnerable to flu - the elderly and infirm - it has nowhere near the 30m doses for a pandemic.
Statisticians believe a flu pandemic is long overdue. They normally occur at fairly regular cycles, but there has not been one since 1968.
The most serious outbreak was the 1918-19 Spanish flu pandemic, which started in China. In 1957, an Asian flu pandemic, thought to have originated in Russia, affected 10-35% of the world's population though mortality was much lower than in 1918. The 1968 outbreak, first detected in Hong Kong, killed around 700,000 people worldwide.
