Officials are to continue testing their theory that the bird flu outbreak at a turkey farm in Suffolk was caused by poultry from a plant in Hungary.
Partly-processed birds from Hungary were being trucked to Suffolk
The government no longer thinks a wild bird brought the virus to the Bernard Matthews farm in Holton.
It says the virus "may well be identical" to a strain found in Hungary earlier this year.
And it says "partly-processed" turkey was trucked from the firm's plant in Hungary to the Suffolk farm each week.
'Poultry to poultry'
The H5N1 virus, which causes bird flu, was found on a Hungarian goose farm near Szentes in southern Hungary last month, and thousands of geese were destroyed.
Earlier this month, tens of thousands of turkeys were culled at the Bernard Matthews farm at Holton after the virus was found there.
The company has a processing plant in Sarvar, in Hungary - about 160 miles away from Szentes.
Turkey meat was being taken from the Hungary plant to a processing plant next to the Suffolk premises.
Deputy Chief Vet Fred Landeg said the "working hypotheses" was that the infection came into the farm through such an import.
Shadow environment secretary Peter Ainsworth said it had always been "unlikely" bird flu had arrived in Suffolk as a result of the wild bird population.
"Bernard Matthews now have some very serious questions to answer about their bio-security," he said
Meanwhile, tests on culled turkeys from three sheds on the Suffolk farm, near the shed in which the virus was first found, also showed strains of H5N1.
Safe to eat
BBC science correspondent David Shukman said the theory raised two questions: "How on earth did the turkeys in Hungary get infected in the first place, and then secondly how did the infection pass from the processing building in Suffolk, to the sheds where the live turkeys were being kept?"
He said the fact that traces of infection had been found in three more sheds, also raised the question: "How did that infection jump from one building to another?"
Bernard Matthews stressed it was co-operating fully with the Defra investigation.
But commercial director Bart Dalla Mura said he would be "very surprised" if Hungary turned out to be the source of infection.
"We do transport meat but we don't move live birds between Hungary and the UK," he said.
"There is no suggestion of any infection at our Hungarian plant and no suggestion of any infection in turkeys in Hungary.
"If we had any concerns about our Hungarian operation we would say so - as we did at Holton. We operate with as much rigour in Hungary as we do in the UK."
The firm added that Bernard Matthews products were perfectly safe to eat. Defra also said the risk to human health remained negligible, and properly cooked poultry was safe to eat.
Defra said poultry is continuing to be imported into Britain from Hungary as long as it is not from restricted areas.
The H5N1 virus does not pose a large-scale threat to humans, as it cannot pass easily from one person to another.
Experts, however, fear the virus could mutate at some point in the future, and in its new form trigger a flu pandemic, potentially putting millions of human lives at risk.