A study, published in the journal Proceedings of the National Academy of Sciences, indicated that an attack could be brought under control using quarantine and targeted inoculation.

While the Government has ordered a stockpile of smallpox vaccine, it is highly unlikely that all or even most of it would be needed in the event of an attack, says the study by Professor Neil Ferguson of Imperial College, London, and Steven Riley of the University of Hong Kong.

As the vaccine causes serious side-effects in 1 in 1,000 people and would kill an estimated 1 in 35,000, widespread inoculation would almost certainly result in more deaths than lives saved.

The only scenario in which mass vaccination would be justified would be an attack abroad which led to smallpox becoming endemic in a country such as the US.

This would lead to a steady trickle of infected people travelling to Britain, making containment by quarantine and targeted jabs harder to achieve.

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