Government vets are investigating an outbreak of an illness which has killed 1,000 turkeys on a farm in Suffolk.
Birds are being tested at a farm near Halesworth, Suffolk
The birds are being tested for a number of diseases, including avian flu, at a farm believed to be owned by Bernard Matthews at Holton near Halesworth.
But officials have stressed it would be premature to say they are looking at a suspected case of avian flu.
The results of preliminary tests by government vets should be known late on Friday night or on Saturday.
A Department of Environment, Food and Rural Affairs (Defra) spokeswoman said: "We are currently investigating a report of unexplained illness on a poultry farm in the Suffolk area."
She 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 symptons of avian flu.
However scientists said the flock had not died as quickly as they would have expected if the deaths were due to the illness.
They also point out that a number of infections can kill turkeys in such large numbers.
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.