The avian flu which killed 2,600 turkeys at a Bernard Matthews farm in Suffolk has been confirmed as the Asian strain of the H5N1 virus.
The virus can be fatal if it is passed on to humans but experts said the outbreak was being contained and posed little danger to people's health.
The Veterinary Laboratories Agency carried out the tests which confirmed the outbreak in Holton.
The slaughter of nearly 160,000 turkeys has begun at the farm as a precaution.
A spokeswoman for the Department for the Environment, Food and Rural Affairs said the birds were being gassed in containers and the operation would continue through the night.
The government department earlier said the flu was the "highly pathogenic" Asian strain, similar to a virus that was found in Hungary in January.
In that incident, the first time bird flu had re-occurred in the European Union since August 2006, a flock of 3,000 geese were killed.
A three-kilometre protection zone and a 10km surveillance zone have now been set up around Holton, which is approximately 27km south-west of Lowestoft.
A much wider restricted zone covering 2,090sqkm is bordered by the A140 to the west and the A47 to the north.
It is the first case on a UK commercial farm of an H5N1 infection.
The strain has killed 164 people worldwide - mainly in south-east Asia - since 2003.
However, the virus is not thought to be able to pass easily from human to human at present.
A spokesman for Bernard Matthews, which runs the farm in Holton, said none of the affected birds had entered the food chain and there was no risk to public health.
So far, all those who have been infected worldwide have come into intimate contact with infected birds.
Fred Landeg, Britain's Deputy Chief Veterinary Officer, said an investigation was under way but the most likely source of the outbreak was wild birds.
He told BBC News that vaccinations for poultry were not currently being considered.
"There are a number of problems with vaccination in that it takes about three weeks to get immunity."
Mr Landeg said the turkeys at the farm had been too young to enter the food chain and no birds or produce had moved off the site.
Dr Maria Zambon, from the Health Protection Agency, said farm workers who had come into contact with infected birds, and those involved in the culling process, would be offered the anti-viral drug Tamiflu as a precaution.
She stressed that nobody had developed symptoms of bird flu following similar outbreaks among farm birds in continental Europe.
Vets were called to the Bernard Matthews farm on Thursday night.
The company said it was confident the outbreak had been contained and there was no risk to consumers.
National Farmers Union president Peter Kendall told BBC News 24 the priority would be eradicating the outbreak.
"[We will be] making sure we get the message across about how well this will be managed and controlled.
"We're encouraging all farmers to be incredibly vigilant, look at their flocks carefully and we do need to reassure consumers that this is not an issue about the safety of poultry - it's completely safe to eat."
Defra has revoked the national general licence on bird gatherings and bird shows and pigeon racing will not be permitted.
Professor John Oxford, a virologist at the London Queen Mary's School of Medicine and Dentistry, said he was confident the outbreak could be contained.
He said: "I don't think it has made any difference as a threat to the human population. The most likely explanation is that a small bird has come in through a ventilation shaft.
"One good thing about this virus is that it's easily destroyed. You can kill it with a bit of detergent."
Dr Oxford also said that while four strains of the H5N1 virus have been identified so far, all are deadly to birds and show potential of being harmful to humans.
In May last year, more than 50,000 chickens were culled after an outbreak of the H7 bird flu in farms in the neighbouring county of Norfolk.
One member of staff at the farm contracted the disease and was treated for an eye infection.
In March 2006, a wild swan found dead in Cellardyke, Fife, was found to have the H5N1 strain of the virus.
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Scene of outbreak
All poultry to be culled
Visitors disinfected and restricted access
3km Protection Zone
Poultry kept indoors and tested
10km Surveillance Zone
No movement of poultry to or from area except for slaughter
Rail transport restricted to non-stopping movements
Bird fairs and markets banned
Increased surveillance of wetland areas
Domestic birds not to share water used by wild birds
Footpath restrictions likely only on free-range farms
People in towns not affected unless they keep poultry.
Isolation of poultry from wild birds
Movements to be licensed