The strains of H5N1 bird flu found in England and Hungary are "essentially identical", scientists have said.
They found the virus that killed turkeys at the Bernard Matthews plant in Holton, Suffolk, was 99.96% similar to one that infected geese in Hungary.
Deputy chief vet Fred Landeg said the most likely transmission route for the outbreak was from poultry to poultry but investigations were continuing.
No evidence of "illegal" movements of poultry products has been found.
The test results came as the Bernard Matthews plant was given permission to resume the slaughtering and processing of turkeys for the first time since the bird flu outbreak.
All the farm's 159,000 turkeys were culled after the H5N1 strain was found there on 3 February.
"The comparison between the UK and Hungarian viruses reveals a high level of genetic match "The analysis of the Suffolk and Hungary strains was carried out by the Veterinary Laboratory Agency (VLA).
VLA chief avian virologist Ian Brown said: "Although other European viruses have shown close relationships to these viruses, these levels of identity are much closer than with other Asian lineage H5 viruses for which data is available, including those isolated from wild birds in Europe in 2005/06.
"The comparison between the UK and Hungarian viruses reveals a high level of genetic match which cannot be said of other European virus strains."
Hungary denials
Mr Landeg said the Hungarian authorities had been informed about the test results.
Investigators in Hungary say they can find no evidence that exported meat was responsible for the Suffolk outbreak.
EU spokesman Philip Tod said investigation was continuing into other possible ways that bird flu could have spread from Hungary to England.
Hungary's chief vet Miklos Suth said no live birds or eggs had been exported from his country.
He denied reports claiming that geese in Hungary infected with bird flu had been culled at the same abattoir in the city of Kecwskemet that had processed turkeys exported to England.
The slaughterhouse where the geese were killed dealt only with geese, while the second abattoir where the turkeys died handled only turkeys, he said.
Laszlo Barany, chairman of the Hungarian Poultry Council, accused the British media of trying to blame Hungary for the Suffolk outbreak.
Co-operation question
Earlier, Mr Tod questioned the level of co-operation provided to the authorities by Bernard Matthews.
Asked if the company had been "completely open", he told BBC Radio 4's Today programme: "I think there are certainly some questions about the levels of co-operation or the speed of co-operation that there has been between the company and the UK authorities.
"On the one hand, it is understandable where you have a situation where you have 160,000 turkeys that have to be slaughtered but, on the other hand, it is important to have full and transparent co-operation with the authorities to handle such an outbreak."
Bernard Matthews said it had always worked closely with the Department of the Environment, Food and Rural Affairs and given it prompt access to all information requested.
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