The type of bird flu found in turkeys on a Suffolk farm is the virulent H5N1 strain, according to government vets.
All the birds at the farm are being culled as a precaution
The virus was discovered on Sunday at Redgrave Park Farm near Diss, where all 6,500 birds, most of them turkeys, are being slaughtered.
A 3km protection zone and a 10km surveillance zone have been set up and the farm is co-operating with vets.
Environment Secretary Hilary Benn said that there might be further undisclosed cases of the disease in the area.
Mr Benn told the House of Commons: "I'm not going to speculate as to whether this outbreak is going to get larger.
"What we're doing is working our darndest to make sure that it stays where it is.
"The most important thing, having locked it down, is to trace the contacts and movements so we can take appropriate action."
A national and a local disease control centre have been established in Bury St Edmunds, with text messages sent to all bird keepers nationwide - especially those in zones on the poultry register.
Mr Benn said that movements within the restricted zones can take place, but not out of it. General licences for low risk movements out of the zone are expected to be available "shortly".
Mr Landeg has urged people with poultry to be vigilant for bird flu
All birds at the affected premises - including approximately 5,000 turkeys, 1,000 ducks and 500 geese - will be slaughtered.
Police officers are at the entrance to the farm, and vehicles are being sprayed with a jet hose.
The BBC's Andrew Sinclair, who is at the farm in Suffolk, said large trucks and gas canisters had been moved on to the premises earlier in preparation for the cull.
It is expected that all the birds will be gassed and then put in sealed containers.
Officials said further local surveillance work would happen before deciding on any culls on neighbouring farms.
The Department for Environment Food and Rural Affairs (Defra) said some 10% of birds in one shed at the farm had died during one night.
Redgrave Poultry Limited, which is operating the farm, has issued a statement in which it said it had continued to fully co-operate with Defra over the outbreak.
It also said the farm was seasonal and prepared birds for Christmas.
"As such, no birds from the farm have been slaughtered for food or sent to customers," the statement added.
Acting Chief Veterinary Officer Fred Landeg said there was "still some uncertainty" over the situation.
"We are at a very early stage of the investigation," he said.
"The initial sequence data suggests that it's closely related to outbreaks in the Czech Republic and Germany, which does suggest a possible wild bird source.
"However, at this stage we are looking with an open mind as to the origin and all potential sources of the origin will be investigated."
Mr Landeg also said there was a lake at the affected site with a number of wild fowl on it and that "no two outbreaks of the disease are ever the same".
"This will not be a quick exercise. This is a particularly challenging site and our priority is to adhere to strict bio-security, and the health and safety of staff on site is paramount."
Earlier, Mr Landeg had said the risk of bird flu spreading was increased during the autumn months because of wild bird migration.
The affected birds were free-range - meaning they had access to the outdoors and may have been at greater risk of catching the disease.
There was a H5N1 outbreak at a turkey farm, also in Suffolk, in February.
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