All of Norfolk and Suffolk, and parts of Essex, have been made subject to avian flu restrictions.
The Avian Influenza Prevention Zone (AIPZ) had been established to mitigate the risk of further outbreaks, the Department for Environment, Food and Rural Affairs (Defra) said.
It follows a number of cases in poultry and wild and captive birds.
The UK’s deputy chief veterinary officer Richard Irvine said bird keepers “must urgently take action”.
Incidents have been confirmed near Hadleigh, Suffolk and in Northwold and Honington, which are either side of the county border near Thetford, Norfolk.
There was also a separate outbreaks at Honington, and others in Bury St Edmunds in Suffolk and Attleborough in Norfolk a week ago.
The new AIPZ was declared following a number of detections of bird flu in both poultry and wild birds across Norfolk, Suffolk and parts of Essex, Defra said.
Defra said the prevention zone would remain in force until further notice and kept under regular review.
Mr Irvine said all bird keepers had to follow “strict biosecurity measures to help protect their flocks, whatever type or size”.
Backyard owners with small numbers of poultry “must take steps to limit the risk of the disease spreading”.
Keepers with more than 500 birds needed to restrict access for non-essential people, Defra said.
Workers on those sites also needed to change clothing and footwear before entering enclosures, and vehicles needed to be cleaned and disinfected regularly.
The H5N1 strain of the avian influenza virus is highly contagious among birds, and spread by close contact with an infected bird, whether it is dead or alive.
Defra said the UK Health Security Agency advised “the risk to public health from the virus is very low”.
Avian flu restrictions put in last month for all of Devon and Cornwall, the Isles of Scilly, and parts of Somerset are still in place.