Bird flu has been confirmed in a flock of 19,500 turkeys at a farm in Boston, the Department for Environment, Food and Rural Affairs (Defra) said.
It is the third confirmed diagnosis of the H5N8 strain of avian flu in Lincolnshire in about four weeks.
Some of the birds at the undisclosed premises in Lincolnshire have died. The rest are due to be culled.
Defra said an investigation was "under way to determine the source of the infection".
A 1.8-mile protection zone and a six-mile surveillance area have been set up around the turkey rearing farm to reduce the risk of the disease spreading.