Two poultry farms, close to a farm in Norfolk infected with bird flu, have found the disease in their livestock.
Further tests will be carried out at the two farms, the government said
The latest infected areas are close to Witford Lodge Farm, where some 35,000 chickens were slaughtered after an avian flu strain was found.
The government said initial tests showed the farms were affected by a less serious strain than the deadly H5N1 which killed a swan in Fife.
Two free range flocks will be slaughtered, officials said.
The Department for Environment, Food and Rural Affairs (Defra) said that preliminary results indicated the farms were affected by the H7N3 strain of avian flu.
The strain found at Witford Lodge Farm, which is in North Tuddenham about 13 miles (20km) west of Norwich, was also the H7 type.
It is virulent among chickens but less of a threat to humans than the H5N1 variant.
Officials said risk to the public remained "extremely low" despite the fact that a poultry worker at Witford Lodge had contracted the virus in the form of conjunctivitis.
A Heath Protection Agency spokeswoman said that no other poultry workers at the farm had shown symptoms of illness caused by H7 avian flu.
Discussing the latest outbreak, a Defra spokesman said: "The two free range flocks will be slaughtered on suspicion of an avian notifiable disease."
A restricted zone has been created, extending 1km from each of the infected premises.
The spokesman added: "The State Veterinary Service is tracing movements and contacts, the necessary surveillance and all appropriate worker protection measures have been put in place."
Debby Reynolds, chief veterinary officer, said: "We still can not say whether either of these two further farms are the index case; further premises may be involved.
"We are investigating whether there any links or movements between the two suspect farms and the confirmed infected premises."
She said the working hypothesis remained that the most likely source of the virus was from another premises or from wild birds.
A Defra spokeswoman said the two new farms which tested positive for avian flu did have the same owner.
The spokeswoman said that one farm had 7,500 chickens and the other had 7,800 chickens.
Last month a swan in Cellardyke, Fife, tested positive for H5N1 - the only confirmed case in the UK so far.
An outbreak of an H7 variation, called H7N7, in the Netherlands led the Dutch government to order the slaughter of more than 30 million birds in 2003.
The 2003 outbreak in the Netherlands infected more than 80 people and led to the death of one vet.
The H5N1 virus has killed more than 100 people in Asia.
But neither strain poses a large-scale threat to humans as bird flu cannot pass easily from one person to another.
However, some experts fear the H5N1 virus could mutate and trigger a flu pandemic, potentially putting millions of human lives at risk.