British officials on Monday confirmed an outbreak of bird flu in turkeys on a farm in eastern England.
The department said the turkeys had tested positive for the H5 subtype of the disease. It was not yet known whether it was the H5N1 strain, which has killed dozens of people around the world.
The Department for Environment, Food and Rural Affairs said a two-mile protection zone and a six-mile surveillance zone had been set up around a farm in Diss, about 100 miles northeast of London, and all 5,000 turkeys, geese and ducks on the premises would be slaughtered.
Within these zones bird movements will be restricted and all birds must be housed or otherwise isolated from contact with wild birds, DEFRA said, adding that the European Union had been informed.
In February, an outbreak of H5N1 bird flu on a poultry farm in the same part of England led to the slaughter of almost 160,000 turkeys.
No definitive source was found for that outbreak, which matched a strain that had earlier infected geese in southern Hungary.
"It appears to have been detected quite early, " Britain's deputy chief veterinary officer Fred Landeg told British Broadcasting Corp. television, discussing the most recent outbreak. "But we have to establish whether there is any other infection in the area, we have to establish the source of the infection, and we have to establish whether the disease has spread any further."
Landeg said the virus might have been spread by wild birds, by animals at another farm, or by the circulation of contaminated poultry products.
Britain's first case of H5N1 flu was in a swan in Scotland in 2006.
Bird flu has killed or prompted the culling of millions of birds worldwide since late 2003, when it first began ravaging Asian poultry stocks. It has killed at least 205 people worldwide, but remains hard for humans to catch. Experts fear it could mutate into a form that spreads easily among people, potentially sparking a pandemic. Experts think most human cases are due to contact with infected birds.
The bird flu outbreak is the latest in a string of bad news for the British farming industry, which is still recovering from outbreaks of foot-and-mouth and blue-tongue disease over the summer. Neither disease affects humans, but both can sicken animals, and the restrictions and slaughter of livestock imposed in their wake have cost farmers millions of pounds (dollars; euros).