Ministers are preparing for the "unlikely event" of a smallpox attack on the UK mainland.

Detailed contingency plans about how the emergency authorities would respond to the release of smallpox came in an interim plan published by the Department of Health and the Ministry of Defence on Monday.

Ministers revealed that 12 dedicated response centres, manned by expert medical teams and the military, are to be established in the event of a biological attack.

The centres would be manned by infectious disease physicians, paediatricians, public health physicians, microbiologists, virologists, acute care and communicable disease control nurses and occupational health staff.

In preparation for such an attack - which could come as a reprisal by Iraq in the event of a war in the Gulf - 350 NHS staff will be vaccinated against smallpox.

A further 350 military personnel, who would work alongside health workers in the event of an attack, will also be vaccinated.

But the government has ruled out plans for mass vaccination throughout the country and intends to adopt a policy of "search and containment".

In the event of a terror attack, ministers say they will attempt to isolate the incident by using "ring vaccination" in the immediate vicinity.

Ministers were keen to stress that the preparations did not mean that an attack was likely.

"Since the tragic events of September 11, the Department of Health has strengthened its plans against any deliberate release of biological agents, including smallpox," said health minister John Hutton.

"We have no evidence of a specific threat of a smallpox attack on the UK. However, it is sensible and prudent to ensure that the NHS can deal effectively with any potential threat.

"There are three components to the department's preparation for response to a possible smallpox emergency: improved vaccine stocks, a plan of action and a cohort of immunised staff who could deal safely with any potential smallpox cases.

"We have already taken action to substantially increase our stocks of smallpox vaccine and we have recently begun a second procurement to add to these stocks."

Defence minister Lewis Moonie said: "We have decided to offer vaccination against smallpox to a small number of specialist military personnel whose specialisation means they face a greater risk of exposure to smallpox.

"This will include those NBC specialists and specialist medical personnel who would form the spearhead of our defensive response, including vaccination teams, in the event of a confirmed, suspected or threatened release of smallpox.''

In a bid to calm the public's fears, Downing Street insisted there was no direct, imminent or suspected threat, but said that Britain had to be prepared for an attack nonetheless.