"Further tests to characterise the virus are underway, in order to ascertain whether or not it is the Asian strain."

The virus was found at a factory farm run by one of Britain's biggest poultry producers, Bernard Matthews, in Holton, Suffolk.

It is the first time the H5N1 strain has been found at a poultry farm in Britain.

The Bernard Matthews farm is in the heart of the country's chicken and turkey farming region and faces having to slaughter as many as 160,000 of its birds as a precautionary measure.

A three-kilometre (1.8-mile) protection zone and 10-kilometre surveillance zone have been set up around the farm, while strict movement controls are in place and farmers are being told to keep poultry indoors.

Britain's environment ministry said it was set to impose further restrictions, adding it was banning bird shows and pigeon racing nationwide following the outbreak.

In a statement, Bernard Matthews stressed that consumers should not be alarmed and said it had strong biosecurity measures in place.

"While Bernard Matthews can confirm that there has been a case of H5N1 avian influenza at its Holton site, it is important to stress that none of the affected birds have entered the food chain and there is no risk to consumers," it said.

And Peter Kendall, president of the National Farmers' Union, told BBC television that consumers should not stop buying poultry because of the scare.

"There's enormous concern, both for the whole farming community, the producers of poultry in the United Kingdom, and making sure we get the message about how well this will be managed and controlled," he said.

"We're encouraging all farmers to be incredibly viglant, look at their flocks carefully.

"We do need to reassure consumers, however, that this is not an issue about safety of poultry. It's completely safe to eat."

Officials announced Friday that they had detected the H5 strain of the virus at the farm, though it was not confirmed as H5N1 until Saturday.

Government vets were called to the farm earlier this week after the death of more than 2,000 turkeys before H5N1 was confirmed.

It is still not clear how the virus was transmitted to the farm, where animals are housed in warehouses.

Professor John Oxford, a virologist at London's Queen Mary's School of Medicine and Dentistry, told the BBC that the "most likely" explanation was that a small bird had come in through a ventilation shaft.

Adding he was confident that the outbreak could be contained, he said: "I don't think it has made any difference as a threat to the human population...

"One good thing about this virus is that it's easily destroyed. You can kill it with a bit of detergent."

In March 2006, a wild swan found in Cellardyke, on the east Scotland coast, was found to have the H5N1 variant of the virus.

The H5N1 strain can be transmitted to humans and has killed more than 160 people worldwide since 2003, most of them in Asia.

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