Defra has said the backyard flock of chickens that were found to have avian influenza on 4 January were not housed.
The outbreak was just 18km from a wildlife park where the H5N8 virus had been discovered in a wild widgeon days earlier.
In government update, officials said the backyard flock of chickens had been mixing freely with Muscovy ducks, highlighting the importance of housing poultry.
See also: Map of UK avian influenza incidences
Defra reminded poultry keepers where possible to house flocks, keep feed out of reach of wild birds and not to allow different species of poultry to commingle.
Gatherings of poultry, such as pure breed shows, also continue to be prohibited.
Earlier this week Defra confirmed it had extended the housing order to 28 February.
Meanwhile, at the turkey farm near Louth, Lincolnshire, hit by H5N8 avian influenza in late December, secondary cleansing and disinfection is underway. No further outbreaks have been detected.
Wild birds with the virus continue to be discovered as far apart as Conwy, north Wales, and Devon, in the South West.
Defra said these findings were expected, given how widespread these birds are across Europe.
Its report added: “We would like to remind all poultry keepers that there are several pathways for the introduction of any notifiable avian disease into a poultry farm: housing alone will not be enough to reduce the risk of some of these pathways.”