Workers were slaughtering 35,000 chickens on a Norfolk poultry farm today after some tested positive for bird flu.

Police officers stood guard at the entrance to Witford Lodge Farm at Hockering, near Dereham, where a strain of bird flu was found in samples from chickens that had been found dead.

Preliminary test results showed that the virus was likely to be the H7 strain, and not H5N1, which is a more virulent strain and a bigger threat to human beings. Further tests would be carried out today at the farm, which produces breeding stock for chicken farmers.

"Restrictions have been placed on the farm and birds on the premises will be slaughtered on suspicion of an avian notifiable disease," a spokesman for the Department of Food, Environment and the Regions (Defra) said.

"When the additional laboratory results are known, further action may be taken." He declined to say how many birds were infected.

Nine potential subtypes of H7 are known. H7 infection in human beings is rare but can occur among people who have direct contact with infected birds. Symptoms may include conjunctivitis and upper respiratory tract problems. On very rare occasions it has proved fatal to human beings.

In 2003 an outbreak of the H7N7 strain in the Netherlands led to the culling of 30 million birds, more than a third of all Dutch poultry, at a cost of hundreds of millions of euros.

The latest alert is of huge significance to the poultry industry in the UK, which is worth about £3 billion a year at retail level. The Government will be particularly concerned about the threat to farms, because it was the H7 avian flu virus that devastated bird populations on a turkey farm in Norfolk in 1992.

Last month a wild swan that was found dead in the harbour at Cellardyke in Fife was found to have died of avian flu. That bird tested positive for the H5N1 version of the virus, which has been responsible for the deaths of more than 100 people, mostly in Asia. Although hundreds of wild birds have been tested in the past few months, the swan is the only case of H5N1 to have been found in the UK.

The Norfolk H7 outbreak comes as research indicates that Britain will need to double the stockpile of antiviral drugs that has been ordered by the Government to stand the best chance of controlling a flu pandemic.

The most comprehensive simulation yet of how pandemic flu would affect Britain has shown that antivirals such as Tamiflu could halve infection rates, but only if enough is available to treat half the population.

While scientists are reasonably certain that there will be another flu pandemic within two decades, it remains unknown whether the H5N1 strain will cause one, or how it would behave should it adapt to humans.

Treatment of half the population with antiviral drugs, which would be combined with school closures, would require about 30 million courses of Tamiflu, compared with the 14.6 million ordered by the Department of Health.

This would allow the drug to be given on a preventive basis to everyone in an infected household as well as to people who fell ill with pandemic flu, which the new model shows to be the best method of containing the disease.

Neil Ferguson, Professor of Mathematical Biology at Imperial College, London, who led the study, said that only France has ordered enough Tamiflu to take this approach.

"It’s hard to be critical of the position adopted so far, as until we published this study it was not clear how large a stockpile would be needed for the best results," he said. "But this does mean it would be wise to order enough antivirals for half the population, and my under- standing is that ministers are actively considering this."

Sir David King, the Government’s Chief Scientific Adviser, said that policy would be reviewed in light of the findings, which are published today in the journal Nature.

"The Tamiflu order at the moment is such that we should have enough for a very substantial population usage in the UK, and we are receiving doses on the order of 900,000 per month," he said.

In the study, Professor Ferguson and colleagues from Johns Hopkins University in Baltimore, Maryland, modelled how a flu pandemic would affect Britain and the US under different conditions.

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