About 1,000 turkeys at a farm in Suffolk have died from bird flu, government vets have confirmed.
The birds tested positive for the H5 strain of avian flu
Vets from the Department of Environment, Food and Rural Affairs (Defra) said the birds had tested positive for H5 avian flu.
It is not yet known if it is the H5N1 strain, which can be passed to humans.
Further tests are being carried out on the birds which died at a farm believed to be owned by Bernard Matthews, in Holton near Halesworth.
Defra said reports from the farm were received late on Thursday night and the premises were immediately placed under restrictions.
"A full investigation began at 0900 GMT this (Friday) morning, with samples being sent to Veterinary Laboratories Agency, Weybridge, for testing," she said.
Sources at Defra have told the BBC that the alarm was raised by the farmer after he noticed "significant mortality" among his flock.
About 80% to 90% of the turkeys in the shed were showing signs of illness - going off their food and general malaise, which are among the symptoms of avian flu.
There are 15 types of bird, or avian, flu. The most contagious strains, which are usually fatal in birds, are H5 and H7.
There are nine different types of H5. The nine all take different forms - some are highly pathogenic, while some are pretty harmless.
The type currently causing concern is the deadly strain H5N1, which can prove fatal 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.