Sir Liam went on television yesterday to appeal for calm. He said that he believed that the likely toll would be much smaller. However, he said that it was a question of “when, not if” the pandemic would occur.

He spoke out after virologists at the Veterinary Laboratory Agency in Weybridge confirmed that the outbreak among poultry in Romania was linked directly to the deadly H5NI virus found in a gull in China. It was not clear whether the disease had been spread by migratory birds or through poultry movements.

As a cull of farm birds was intensified in Romania, experts were waiting to see if the virus had infected birds in neighbouring Bulgaria. Panic about a pandemic led to 20,000 face masks being sold in a day in Serbia.

Sir Liam does not expect an epidemic to happen this winter, but he said: “We can’t make this pandemic go away because it is a natural phenomenon. It will come, but what we can do is to limit its impact.”

The disease will be discussed in relation to trade at a meeting of European foreign ministers this week. The subject was said to be off the agenda of the meeting today of environment and farming ministers.

Contingency plans for an epidemic in Britain, however, make clear that the country could come to a virtual standstill. At the peak of an outbreak, foreign travel would be placed out of bounds, with ports and airports closed. Many people could be virtually housebound with the closure of schools, town halls and places of entertainment, and the cancellation of sporting fixtures.

Key workers would be given priority for a vaccination, including NHS staff. Vulnerable groups such as elderly and ill people and those living in closed communities would also be vaccinated. The full plan is being reviewed by Patricia Hewitt, the Health Secretary, with an announcement due on Thursday.

There are no immediate plans to order farmers to keep poultry indoors. However, wildlife experts are convinced that the disease will first show itself in poultry. Dr Ruth Cromie, a water bird biology research manager at the Wildfowl & Wetlands Trust, said: “This focus should be on all poultry movements.”

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