The Department for Health confirmed it had bought vaccines against the disease but refused to say how many or at what cost.

But a spokesman told BBC News Online there was no current credible terrorist threat to Britain.

The Evening Standard's website is reporting that government bought 30 million anti-smallpox vaccines at a cost of £32m to bolster defence against the disease.

Germ warfare

Within 12 months health chiefs will have enough vaccine to protect at least half the population - more if the doses are divided up, it says.

The deal revealed on Friday is with the British company Powder-Ject Pharmaceuticals.

Both the US and Russia have already stockpiled anti-smallpox vaccines in the wake of 11 September amid fears of germ warfare.

A Health Department spokesman told BBC News Online: "As part of the government's continuing vigilance against international terrorism we have acquired supplies of smallpox vaccine.

"There is no credible threat but it is important for the government to take all necessary steps to ensure the protection of the population."

"For obvious national security reasons we can't discuss these arrangements in detail."

Infectious

Smallpox has a fatality rate of 30 per cent or more when used as a biological weapon, according to US public health research.

While it was effectively eradicated worldwide 25 years ago, it is feared as the most devastating of all infectious disease.

Scientists in the United States recently found 90 million smallpox vaccines, increasing the country's stock six-fold.

The doses manufactured by a French company, Aventis Pasteur were discovered at its plant in Swiftwater, Pennsylvania.

The US and Russia kept supplies of the virus itself following the eradication of the disease in humans.