The Government has bought the wrong vaccine to protect the country from the threat of a smallpox attack by terrorists, it was claimed today.

A leading American authority on the subject said the Lister vaccine ordered by the Department of Health had not been proven to work against the so-called “battle-strain” of virus obtained by Iraq.

Steve Prior, of the Potomac Institute who has carried out new research on the smallpox threat, told The Times that he believed Britain’s vaccine choice was “indefensible”.

He said the US government had selected an alternative vaccine developed by the New York City Board of Health which had proved successful against endemic smallpox in India from which the “battle-strain” was developed.

“I was very surprised ... for the Department of Health to suddenly come out and say ‘we know something about the threat that nobody else knows that means we have to have Lister’ was very surprising,” Dr Prior said.

“When you are making decisions of this magnitude, you cannot afford to make the wrong one and you certainly cannot afford to make it for the wrong reasons ... I think it is indefensible.”

However the Department of Health, which placed the £28 million order for the vaccine, insisted that it was acting on expert medical and scientific advice.

A spokesman said that it had chosen the same vaccine as other European nations.

“The decision to purchase the Lister vaccine was based on the expert medical and scientific advice that we and the Ministry of Defence had,” the spokesman said.

“As far as we are concerned it is the best we can get which is why we bought it.”