The bird flu outbreak at a Bernard Matthews farm in Suffolk may be linked to imports from the firm's plant in Hungary, a government vet has said.
Partly-processed birds from Hungary were being trucked to Suffolk
The pathogenic H5N1 strain was found on a Hungarian geese farm in January.
Deputy Chief Vet Fred Landeg said imported "poultry product" was a possible route of infection.
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.
'Poultry to poultry'
The Department for Environment, Food and Rural Affairs said preliminary tests showed the Hungary and UK viruses "may well be identical".
Geese on the infected farm near Szentes in southern Hungary were destroyed last month, after the EU's first case of bird flu for about six months was found.
Earlier this month, tens of thousands of turkeys were culled at the Bernard Matthews farm at Holton after the H5N1 virus, which causes bird flu, was found there.
The company has a processing plant in Sarvar, in Hungary - about 160 miles away from Szentes.
Defra confirmed "partly-processed" turkey had been transported by lorry from Hungary to the Suffolk farm each week up to the time of the Suffolk outbreak.
The turkeys were taken to a processing plant next to the premises which became infected.
Mr Landeg said the "working hypotheses" was that the infection came into the farm through such an import.
He said the latest tests "seems to indicate that this is an infection that has been passed from poultry to poultry", rather than from wild birds.
BBC science correspondent David Shukman said the news 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?"
Safe to eat
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.