The Department of Health said that while there was no evidence of a specific threat it was carrying out "intensive planning" just in case.

Key health workers, including doctors and nurses, will be first to be offered the vaccine. They will form the first line of defence in any outbreak, caring for those taken ill in isolation.

We should have in place enough vaccine to vaccinate on a mass population basis if necessary

Sir Liam Donaldson Chief medical officer

In the event of an attack, a limited vaccination programme would be introduced in the affected area.

A mass vaccination programme would only be considered if there were a number of outbreaks.

Plans in place

Sir Liam Donaldson, the chief medical officer for England, said it was important that structures were in place to deal with every eventuality.

"We believe we should have plans in place both to search and contain, with limited numbers of people being vaccinated around the source of the outbreak," he said.

"We should also have enough of the vaccine in place to administer on a mass population basis if necessary."

A spokeswoman for the Department of Health insisted there was no increased risk of attack.

"We are just making sure that we are prepared if there is one," she said.

"The department has been talking about this since September 11 last year because that brought it more to the surface - lots of other countries around the world are doing the same thing."

Major killer Once a major killer, smallpox is now confined to the laboratory.

The disease claimed the lives of 300 million people in the 20th Century, before a vaccine finally wiped it out.

It is good news that the vaccine is going to be bought

Dr Vivienne Nathanson, BMA

However, last year's September 11 attacks on the United States have raised fears that smallpox could be used by terrorists.

The vaccine while effective is known to cause serious side-effects in some people, in particular those with weak immune systems caused by HIV or cancer.

Recent studies have suggested that a small proportion of patients may suffer severe, even life-threatening, reactions to the vaccine.

Medical support

But the British Medical Association welcomed the move.

Dr Vivienne Nathanson, its head of science and ethics, said: "We should be relieved.

"It is good news that the vaccine is going to be bought so if there were to be such an attack we would have the facility to vaccinate everyone who would be at risk and indeed the whole population."

However, Dr Ian Gibson, Labour MP and chairman of the Commons science and technology committee, said the government should be vaccinating the entire population and not just healthworkers.

"We should move as quickly as possible to innoculating the whole population," he told the BBC.

Dr Gibson added that a biological attack on the UK "will happen eventually".

He said: "I'm sure somebody will try it."

Two weeks ago the US Government unveiled its own plans to vaccinate the entire American population - 228 million people - against smallpox within seven days of any terrorist biological attack.