Some 35,000 chickens at a poultry farm in Norfolk are being slaughtered after dead birds tested positive for a strain of bird flu.
Farmers have been told to keep up high standards of biosecurity
The dead chickens were found on Witford Lodge Farm in North Tuddenham, about 13 miles (20km) west of Norwich.
The government's chief vet said it was likely to be the H7 strain, virulent among chickens but less of a threat to humans than the H5N1 variant.
Dr Debby Reynolds added she did not know where the flu had come from.
Last month a swan in Cellardyke, Fife, tested positive for H5N1 - the only confirmed case in the UK so far.
Dr Reynolds said there was no evidence of the H5N1 strain in the chickens from the Norfolk farm.
Dennis Foreman, director of Banham Poultry Ltd, the company which owns Witford Lodge Farm, said the number of dead birds had been "minimal".
"As a company we don't want this but at the end of the day it happened and we have got to deal with it professionally," he said.
"With the help of Defra we think we are in safe hands."
An outbreak of an H7 variation, called H7N7, in the Netherlands led the Dutch government to order the slaughter of more than 30 million birds in 2003.
The cases in Norfolk were found in samples taken from chickens on the farm, the Department for Environment, Food and Rural Affairs (Defra) said.
The birds at the farm are used to produce eggs that are hatched elsewhere, to create more chickens.
Movement restrictions are in place on the affected farm, with no movements allowed on or off the premises except under licence from the veterinary inspectors, a Defra spokesman said.
The same restrictions apply at 30 other premises in Norfolk owned by the company as a precautionary measure.
The spokesman said the possibility of introducing an exclusion zone was not being ruled out but any decision would await the findings of tests.
Dr Reynolds said poultry should be "fed and given water indoors and the highest standards of biosecurity should be maintained at the moment".
Earlier she told the BBC she expected further test results to reveal more about the bird flu strain over the next 24 hours.
"Those results will allow us to decide whether it's the highly pathogenic dangerous form to birds, which kills a lot of birds, or the low pathogenic which is a much less serious infection," she said.
She said no decision had been taken on whether to destroy poultry on neighbouring farms, adding the slaughter was a "highly precautionary" measure to ensure there was no spread of the disease.
"The other investigation is, where did it come from? And at this stage we don't know the answer to that," she said.
Paul Leveridge, who has a farm with around 15,000 ducks just two miles from Witford Lodge, said it was a stressful time for farmers.
"Every poultry farmer at the moment is worried, especially as this is now in Norfolk, which is the heart of the poultry industry," he said.
The National Farmers' Union has said it expected restrictions to be enforced in the area and said the farm should be compensated for the loss of flock.
Defra has stressed that there was no confirmation the virus had health implications for humans.
The 2003 outbreak of H7N7 in the Netherlands infected more than 80 people and led to the death of one vet.
The H5N1 virus has killed more than 100 people in Asia.
But neither strain poses a large-scale threat to humans as bird flu cannot pass easily from one person to another.
Humans also have to have extremely close contact with infected birds, particularly their faeces, in the first place to catch it.
However, some experts fear the H5N1 virus could mutate and trigger a flu pandemic, potentially putting millions of human lives at risk.
Prof Hugh Pennington, a professor of bacteriology at Aberdeen University, said while the H7 strain was "nasty for the birds", it was "not a public health threat to humans".
"It's basically a virus that kills chickens and has been around for many, many years.
"It's there in wild birds probably, circulating throughout the world and occasionally we get an outbreak in this country," he told the Today programme.
The policy of killing the flock was the best option for controlling the virus, he said.
Health Protection Agency East Anglian director Dr Sue Ibbotson said about a dozen workers - plus staff involved in the destruction of the birds - were being monitored and offered anti-viral drugs.
"People who became infected could suffer flu-like symptoms or eye irritations. There is no need for panic," she added.