But limiting the vaccine's use to outbreaks of the deadly disease was heavily criticised by the Labour chairman of the House of Commons select committee on science and technology, who said it could cause widespread panic.

"I don't think we can assume that it is only necessarily going to be the health workers who are subjected to the attack," said Ian Gibson. "People will say - why not me as well? You could set off fear and panic by protecting just one sector of the population.

"The only sensible way to alleviate people's fear - because they will be frightened - is to vaccinate everybody."

He also questioned the timing of the revelation, by the chief medical officer, Sir Liam Donaldson, that contingency plans were being made to vaccinate. "Why now?" asked Dr Gibson. "It is just building up this Iraqi scare that Madam Death is waiting in the wings. Terrorism is always on the agenda, from the Tokyo underground days. Why now? is the question, and I shall be asking it in the House of Commons."

Britain is following in the footsteps of the United States, where there has been enormous pressure on the government to vaccinate the entire population. Two weeks ago, the Centres for Disease Control announced detailed plans for inoculating 288 million Americans against smallpox, within days of an attack. It has said that pre-event inoculation is still under consideration, but there are no plans yet to protect anyone except key medical and emergency personnel before an outbreak of the disease.

The UK government says its position is similar. "If we have an attack, we need some people to be immune. Therefore we need to vaccinate some people in advance. We're considering who that might be and how many of those people there might be," a Department of Health spokesman said.

The position of Sir Liam is that the vaccination of health workers has to go ahead. "It is when rather than whether," he has said.

The policy in the event of an attack is to "ring fence" the outbreak, by rushing in stocks of vaccine and immunising all contacts of those people who have shown signs of the disease.

The side effects of the vaccination, which include fatal reactions, tip the balance against universal immunisation unless there is a confirmed attack. John Oxford, professor of virology at the London hospital medical college, said one in a million people used to die during the mass vaccination campaign which lasted from around 1948 to 1971.

"Immunising 50 million, you might get 50 deaths, mainly in children with eczema or these days people with HIV," he said. "In those days, when you could have died of smallpox, you took the risk."

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