All 10,500 turkeys will be culled at a farm in North Yorkshire, in England after bird flu was confirmed at the site. The H5N8 strain of avian influenza was found at a turkey fattening premises near Northallerton on Saturday.
The birds will be humanely culled to stop the disease spreading, the Department for Rural Affairs (Defra) said, as it investigates the source.
Risk to public health from the virus is very low, Public Health England (PHE) added.
A temporary control zone is currently in place -a 3km protection zone immediately around the location of the outbreak and a 10km surveillance zone around that.
Defra's chief veterinary officer, Christine Middlemiss, said: We are urgently looking for any evidence of disease spread associated with this farm to control and eliminate it.
Bird keepers should remain alert for any signs of disease, report suspected disease immediately and ensure they are maintaining good bio-security on their premises.
The disease can spread to poultry and other captive birds when wild birds migrate from mainland Europe during winter, Defra added.
Routine health advice is being offered to those working on the farm.
Dr Gavin Dabrera, consultant in Acute Respiratory Infections at PHE, said the World Health Organization has never confirmed any cases of H5N8 in humans to date.
The Food Standards Agency said properly-cooked poultry and poultry products, including eggs, remain safe to eat.