The outbreak -- the H5 strain of the virus and not necessarily the lethal H5N1 Asian type -- was found on a farm near the town of Diss, said a spokesman for the Department for Environment, Food and Rural Affairs (Defra).
A three-kilometre (1.8-mile) radius protection zone and a 10-kilometre surveillance zone were imposed around the farm in the county of Suffolk, where there was an outbreak of H5N1 in January.
"It is H5 but we don't know which strain," the spokesman told AFP, saying further tests were being carried out to find out if it was the lethal H5N1.
"Full confirmation of results, including whether or not this is H5N1 and whether the strain is high or low pathogenic, will follow," added a Defra statement.
All birds on the farm, including turkeys, ducks and geese, will be slaughtered, the spokesman said.
"We are also urgently considering with ornithological and other experts what wider measures may be needed," said Defra, adding that the European Commission had been notified.
The outbreak is the first case of the disease in Britain since May, when a child became infected with a low-risk strain of bird flu in Wales.
In January some 159,000 turkeys were killed as a precaution at a plant near Holton in Suffolk after an outbreak, which led to some countries to impose import bans on British poultry.
That case followed the detection of the H5N1 strain among geese in Hungary, the first such outbreak within the European Union since mid-2006.
The H5N1 strain first emerged in Asia in 2003, and has caused some 205 deaths in humans, with Indonesia and Vietnam among the worst hit countries, according to World Health Organization figures.
"Everybody needs to be concerned. This is avian influenza. We are asking every poultry keeper to be vigilant," deputy chief veterinary officer Fred Landeg told Sky News television.
"We will be looking at the movements on to the premises and off the premises of birds and movements of people, vehicles and things, to see whether there is another origin somewhere in the country or whether the disease could have spread," he added.