The Department of Health stressed on Tuesday that it had no plans to introduce a mass vaccination programme against the disease.

Ministers said the vaccine would only be offered to key health and military staff, even though the government is on course to stockpile 60 million doses of the drug.

The public should be reassured that we have a very effective plan in place should the worst happen

Sir Liam Donaldson, Chief Medical Officer

But a report, published by the Department of Health, suggested the policy could change if public demand for protection from smallpox increased.

The report, entitled Guidelines for Smallpox Response and Management in the Post-Eradication Era, sets out for the first time how the UK would deal with the threat of smallpox attack.

Alert levels

It sets out five levels of alert to deal with the threat. This ranges from level 0, the present position where there is no creditable threat of a release, to level 4 where a large outbreak has occurred at various locations around the country.

Under the plans, a mass vaccination programme will only be considered at level 4.

At all other levels the priority will be to protect health workers who will be responsible for treating patients with the disease and trying to control any outbreaks.

The effects of smallpox can be terrifying

But the report acknowledges that the policy may need to be revised.

It states: "Public demand may also influence the decision to implement mass vaccination."

But it adds that a good public relations strategy should prevent this from happening.

"General public demand for vaccination is not necessarily inevitable if public relations are good from the outset," the report states.

The government has stressed that there is no evidence of a specific smallpox threat against the UK.

Health Minister John Hutton said the measures being introduced were "prudent" and would make sure the NHS could cope in the event of an attack.

Mr Hutton announced that approximately 700 key health and military workers would receive the vaccine by the end of this month.

Response groups

These workers will be part of new 'Regional Smallpox Response Groups, which will form the front line against any attack.

These will consist of infectious disease specialists, paediatricians, public health doctors, microbiologists and virologists. Specialist disease control nurses will also be on the teams.

Although there has been some pressure both in the US and UK for a precautionary mass vaccination programme prior to any attack, many experts believe this is not a good idea.

This is because the smallpox vaccine, like any other, will cause side-effects, perhaps even fatal ones, in a small proportion of patients given the jab.

Mass vaccination of millions would inevitably lead to the deaths of dozens.

Instead, by targeting key workers, the risks can be minimised.

England's Chief Medical Officer Sir Liam Donaldson said the current policy was the right one.

He told the BBC: "The public should be reassured that we have a very effective plan in place should the worst happen.

"Part of this is ensuring that our 'first responders' are vaccinated and protected against the disease."

The Conservatives have urged the government to be honest with the public about any possible threats.