An investigation into the outbreak of the lethal H5N1 strain in exotic birds at an Essex quarantine centre showed that most other birds, including chickens, escaped infection. This means that the virus could be much less contagious in birds in Britain than previously suggested.

The news follows reports that China is to vaccinate billions of poultry after the country reported its ninth outbreak of the virus in a month.

There were fears that continued contact between infected birds and humans could result in the H5N1 virus mutating into a form that could be transmitted among humans.

Senior vets and epidemiologists at the Department for Environment, Food and Rural Affairs now believe that the disease was brought to Britain in a consignment of 101 mesias, or finches, from Taiwan. It was previously thought that the first source of the virus in Britain was in a blue-headed pionus parrot from Surinam.

A total of 53 mesias died in quarantine at a centre near Southend-on-Sea run on behalf of the importers Pegasus Birds, though it is not certain how many died of the flu strain.

There are concerns that some sick birds were incinerated without testing. Incineration, however, can be done only on the instructions of a vet but the vet involved with Pegasus has not yet been interviewed.

The Government report sets out a factual account of the incident although experts admitted that they could not be definitive about the source of the virus because tissue from the parrot and a sick mesia were mixed for testing. However “on the balance of probabilities”, they blamed the mesia.

Their view is underlined because no evidence of the H5N1 strain was found in any of the other 148 birds imported from South America. Even though two black-headed caiques and a green-winged macaw were found dead in the batch on arrival on September 16, samples tested at Weybridge were clear.

A further 34 caiques and six blue-headed pionus also died during quarantine. Some have been frozen or refrigerated and others incinerated. All samples tested have been negative.

Stringent testing by experts at the Veterinary Laboratories Agency in Weybridge also found no evidence of the H5N1 strain in other birds from the Taiwanese consignment. There were 65 mynah birds and 20 black-throat laughing thrushes included in the batch which arrived at Heathrow on September 28.

Crucially, however, there was no infection found in four British chickens used as “sentinel” birds in the quarantine premises. The presence of these birds is compulsory to act as an early warning to authorities about any outbreak of avian flu and Newcastle disease, the two notifiable diseases.

Ministers and senior vets believe that the scientific findings are of huge international importance and show that the strain is not easily contagious between various bird species, including migrating birds.

Ben Bradshaw, the Animal Health Minister, said: “Given the fact that these finches had this strain in close confinement with birds, including chickens, and none of those birds got the infection, I think that will be found to be pretty significant around the world.”

In China, Jia Youling, the state chief veterinarian, did not say how exactly the vast vaccination programme would be carried out, how long it would take or how much it would cost. Health authorities are now producing more than 100 million doses of bird flu vaccines a day, but some areas are still reporting shortages, the ministry of agriculture said.

Mr Jia said that China now has about 5.2 billion birds as livestock and raises about 14 billion each year.

The first case of the H5N1 strain occurred in a Chinese goose in 1996.

Mr Jia said that the nine outbreaks of the disease reported across China this autumn “have been basically brought under control”. In each case, China has ordered all poultry within a 3km-radius (1.8 miles) to be destroyed. Several million birds have been killed so far.

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