Acting chief veterinarian Chief Veterinary Officer Fred Landeg 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.
Bird flu's return to Britain is yet another blow to farmers, who are already struggling after herds were hit this year by foot-and-mouth disease and bluetongue.
If the bird flu spreads, it could devastate the hugely profitable Christmas trade in poultry.
A three-kilometer (two-mile) protection zone and a 10-kilometer (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.
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