The Government has given warning that wild birds migrating to Britain over the next few weeks may be carrying the H5N1 strain of avian flu.

Debby Reynolds, the Chief Veterinary Officer, is ready to order birds to be kept indoors if farms are deemed to be at risk.

A risk assessment published by the Department for Environment, Food and Rural Affairs states that the highest risk is from birds that have passed through the Czech Republic, Germany and France. Last month there were three outbreaks of the flu strain in the Czech Republic, and three cases in southern Germany. There was also an outbreak in France this month and experts expect more cases.

The greatest threat is believed to be from the blackheaded gull, which returns to Britain from the area around the Baltic Sea. About 50,000 to 100,000 pairs of these gulls breed in the Czech Republic before leaving for other parts of Western Europe, including Britain. These birds could appear this month.

The other imminent threat is from the mallard, which will start returning to Britain next month. Mallards breed in large numbers throughout Europe, with 25,000 to 45,000 pairs in the Czech Republic and 200,000 to 400,000 pairs in Germany.

The experts do not yet know which bird species is spreading the virus but they believe that several species will move to Britain from infected areas.

Customs officers are making secret checks on flights from affected areas. Sniffer dogs are being used at Heathrow and Gatwick to identify illegal imports of poultry meat, cooked and raw, and eggs that may carry the H5N1 virus. Among the flights, passengers and baggage being screened for possible smuggled food are those from China, Russia, Egypt, Thailand and Vietnam.

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