The return of the disease, which was first confirmed in Britain in February, is yet another blow to farmers, who are already struggling after herds were hit this year by foot-and-mouth disease and bluetongue.
Acting chief veterinarian Chief Veterinary Officer Fred Landeg said the source of the outbreak had not yet been identified. He said it was closely related to the strains found in the Czech Republic and Germany over the summer.
Officials said earlier that about 5,000 free-range turkeys, 1,000 ducks and 500 geese on the affected farm were being culled.
If the bird flu spreads, it could devastate the hugely profitable Christmas trade in poultry.
A two-mile protection zone and a six-mile surveillance zone were created around the infected farm, and further restrictions were imposed throughout Suffolk and much of the neighboring county of Norfolk.
The alarm was raised Sunday, after a rise in death rates among the birds, which are owned by poultry producer Gressingham Foods, based in Woodbridge, Suffolk.