The Government is preparing for a mass smallpox vaccination in the event of a bio-terrorist attack.

Chief medical officer Sir Liam Donaldson has outlined the Government's plans for vaccinating the entire British population.

But the Department of Health stresses that the plans do not mean there is an increased risk of attack.

Under the plan, key health care workers in each area of the country would be vaccinated as a precautionary measure so they could respond to any outbreak.

If an outbreak did occur the population in that area would be offered vaccination. Only if there were numerous outbreaks would mass vaccination be considered.

Sir Liam said: "We believe we should have plans in place both to search and contain with limited numbers of people being vaccinated around the source of the outbreak, but also we should have in place enough vaccine to vaccinate on a mass population basis if necessary."

Previously the Government has only talked about plans for a limited vaccine programme to isolate any new outbreak.

Sir Liam told the BBC's Ten O'clock News: "It's when rather than whether and it is who should be vaccinated, because a proper counter plan to a smallpox attack would involve having a group of essential workers who were immune to the disease through vaccination."

A Department of Health spokeswoman says the Government is just making sure that a contingency plan is in place.