British officials now believe the foot-and-mouth outbreak affecting farmers in southern England originated at a vaccine lab.
There is a "real possibility" human movement spread the virus, said the government report released Tuesday.
The possibility that England's recent flooding woes could have triggered the outbreak was dismissed as "negligible," said the government's Health and Safety Executive.
Environment Secretary Hilary Benn said the report brings the fight against the devastating agricultural disease "a bit further forward."
Last Friday, infected animals were discovered at a farm about six kilometres from the Pirbright vaccine laboratory. The government's Institute for Animal Health, or IAH, and a private pharmaceutical company, Merial Animal Health, the British arm of Duluth, Ga.-based Merial Ltd., share the facility.
The unusual strain that has been detected in the infected cattle is the same one contained in the lab. Merial said it was assessing the report, but had previously said it had found no evidence to suggest biosecurity had been breached.
Authorities confirmed Tuesday a second herd had been found to be infected. They have already been culled.
Those 50 cattle were within the initial three-kilometre-radius protection zone set up Friday around a farm in Surrey, where the first group of infected cattle was found, 50 kilometres southwest of London.
Laurence Matthews, who owns the farm where the second infected herd grazed, said the discovery is raising fears the virus could spread across the local rural community.
"We were starting to think this virus had been contained and maybe we were going to be getting back to normality in a few weeks," he told British Broadcasting Corp. radio.
"Now this has set us back again and most farmers, and I've been speaking to a few, are very, very scared."
Many fear a repeat of scenes from 2001, when seven million animals were culled and incinerated on pyres dotted across the landscape. The crisis devastated agriculture and rural tourism in Britain.
Frustration, anger
Peter Kendall, president of the National Farmers Union, said Tuesday's findings "add to the frustration and anger" of farmers.
"I have spoken to many farmers over the last few days who are absolutely horrified that the source of this outbreak could be from Pirbright," he said.
So far, no cases have been detected outside a 10-kilometre zone around the Pirbright facility.
Foot-and-mouth disease affects cloven-hoofed animals including cows, sheep, pigs and goats, but does not typically affect humans.
Britain's Chief Veterinary Officer Debby Reynolds said the strain found in the first outbreak matched samples taken during Britain's 1967 outbreak of foot-and-mouth disease. The strain had not been seen in animals for a long time, but was used to produce vaccines, she said.
The European Commission endorsed Britain's ban on the export of livestock, meat and milk. London also halted movement of cattle, sheep, goats and pigs nationwide to prevent the virus's spread.
Imports of British pigs and pork products have been banned by the United States, Japan, Russia and South Korea in response to the outbreak.
Chuck Strahl, Canada's agriculture minister, told Â鶹´«Ã½net that "we worked quickly and decisively with CFIA to close our borders to any agricultural products that could affect us here."
Canada has been in contact with the U.K. to offer assistance and be kept informed of updates, he said.
Canada has only imported bovine sperm in the past three months and not any consumer products, he said. "It looks like there's nothing in the system ... that could be a problem for us."
He noted the U.K. has suspended exports, "so between them and us, nothing will be coming into Canada. You just can't mess around with this, so that's why we've taken very stringent measures here."
With a report from CTV's Tom Kennedy and files from The Associated Press
