The latest theory is that a free-range egg company, which kept 15,300 chickens outdoors on two farms, is now the likely source of the infection, probably after some contact with an infected wild bird.

More than 50,000 chickens have now been culled on Norfolk Road Farm and Mowles Manor Farm at North Tuddenham, owned by Geoffrey Dann and his son, Simon.

Blood samples from birds on their farm showed that they had been exposed to the H7N3 virus as long ago as four weeks.

This company is less than half a mile from Banhams chicken breeding farm at Whitford Lodge, where 35,000 birds were culled last week after the low pathogenic avian flu strain was confirmed in the county. Birds on the free range unit, however, suffered only a mild form of the flu and none died from the infection. They had to be culled because H7N3 flu is a notifiable disease.

It is still unclear how the virus was transported from the egg farm to the Banhams chicken farm, where it killed some 400 chickens and triggered a drop in egg production by other birds.

Such a reaction to avian flu is expected on intensive commercial units where there are large numbers of birds who live indoors at close quarters.

State vets have been unable to find a link between the egg and the chicken farms. An infected wild bird, or its faeces on a worker’s foot, or vehicle, are still thought to be the most likely routes for the infection.

Farmers around the country have also been reporting suspicious signs of sickness or a fall in egg production and the Veterinary Laboratory Agency, in Weybridge, Surrye, is testing numerous samples.

So far, however, all tests have been negative.

Officials at the Department for Environment, Food and Rural Affairs are determined to ensure that the country’s

£600 million-a-year chicken export trade is maintained. Ministers will today make representations to the Japanese Embassy and Hong Kong High Commission, in London, over their trade bans against Britain.

Japan has halted all trade in eggs, chicken and breeding birds from Britain, and Hong Kong has banned produce from Norfolk.

Such bans are in breach of international law, however, and can be imposed only if a highly pathogenic bird flu strain is present. So far, government vets have found only the less virulent H7N3 strain.

• International quarantine measures, such as restricting air travel from countries with a serious influenza outbreak, would do little to halt the spread of a pandemic, a study by British scientists has found (Mark Henderson writes).

  A strain of flu that passed easily from person to person would move around the world more quickly than it could be detected, making airport closures largely ineffective, according to computer simulations by the Health Protection Agency. For restrictions on air travel from infected countries to delay a pandemic, it would be necessary for almost all travel to be stopped immediately as soon as the virus emerges, scientists found.

  The findings, published today in the open-access journal Public Library of Science Medicine, suggest that the costs of restricting international air travel in a pandemic are likely to outweight the benefits.

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