The avian flu virus was found on Tuesday in laying hens at the farm in Banbury, and all birds on the site were slaughtered.

Further tests are in progress to identify the exact type of the virus, while an investigation is under way to find where the disease came from.

While the H7 strain has been found in Britain on several occasions before now, this is the first time it has been identified in the highly pathogenic, or deadly, form.

Following the discovery of bird flu, a temporary control zone consisting of a 1.9-mile (3km) inner zone and a 6.2-mile (10km) outer zone was established around the infected premises.

In the inner zone, poultry must be housed and kept isolated from wild birds, and across the whole zone movement of birds and bird gatherings are banned.

Following confirmation that H7 bird flu had been found, Chief Veterinary Officer Nigel Gibbens said: "I would stress the need for poultry keepers to be extremely vigilant, practise the highest levels of biosecurity and report any suspicions of disease to their local Animal Health Office immediately."

The Health Protection Agency said the risk to public health remained low, while the Food Standards Agency said there were no implications for humans eating poultry products.

Dr Colin Butter, of the Institute of Animal Health, said highly pathogenic H7, like the deadly H5N1 bird flu strain which has been found on several occasions in the UK, would cause "rapid and very high mortality" among poultry.

Neither type spreads easily to humans, requiring close contact with infected birds. But while H5N1 causes high death rates in humans when they do catch it, H7 does not pose a serious health threat, he said.