Farmers received a devastating blow last night as Defra confirmed the outbreak of avian flu at a farm on the Norfolk/Suffolk border was the deadly H5N1 strain.

Defra has enforced a three kilometre protection zone, a 10 kilometre surveillance zone and a wider restricted zone, covering the whole of Suffolk and most of Norfolk, around the farm.

The outbreak comes at a crucial time for farmers who are preparing for the Christmas rush for turkeys. As many as five million birds within a six-mile radius of the farm and 25 million in the wider restricted zone could be at risk, according to reports in the Daily Mail.

Investigations are being carried out to find the source of the virus. The virus could already be endemic in the wild bird population, according to The Times, while the Guardian suggested investigations were centred on an ornamental lake at a Suffolk stately home where turkeys from the farm had mixed with wild birds.

The H5N1 strain hit a Bernard Matthews farm in Suffolk in February this year, leading to the slaughter of 160,000 birds.

“We have faced H5N1 once already this year, but there is still significant uncertainty surrounding this outbreak,” said acting chief veterinary officer Fred Landeg.

“Swift reporting of disease and stringent biosecurity is essential to controlling this disease and we are working to our established contingency plans,” he added.