Initial tests show the latest outbreak of foot-and-mouth disease is the same strain as that which infected herds last month, the BBC has been told.
A 10km control zone has been set up around the site
Scotland's Environment Secretary Richard Lochhead said first results from tests on cattle in Egham, Surrey, "pointed in that direction".
The government is facing criticism that it was too quick to declare Britain foot-and-mouth free last week.
A movement ban is in place, and the EU has stopped animal exports.
Mr Lochhead told BBC Scotland that his chief veterinary officer had confirmed to him that the foot-and-mouth strain was likely to be the same as the one that escaped from a government laboratory near Pirbright in August.
The infected cattle in the latest outbreak come from a farm 10 miles from the laboratory, and the disease was confirmed only a week after the previous restrictions on cattle movements were lifted.
A 10km (6.2 mile) control zone has been set up around the Surrey site near Milton Park Farm.
Cattle and pigs at another farm nearby were also being slaughtered on suspicion of infection, Defra said.
Chief vet Dr Debby Reynolds said up to 300 cattle were being culled on the farm, and a "small number of pigs".
The president of the National Farmers' Union has spoken of "the real state of despair" amongst his members.
Peter Kendall said this latest outbreak was "much worse" than last month's, because it has come at a time when livestock farmers need to move their animals, and send them to market.
Mr Kendall was attending an emergency meeting of politicians, animal health experts and officials to plan the government's next response to the outbreak.
Protection zone - all premises containing livestock will be inspected by vets, and will be subject to restrictions on movements of animals, animal products, some vehicles, and some activities.
Surveillance zone - restrictions on movements of susceptible animals, animal products, manure, and some activities. No restrictions on the movement of non-susceptible animals, such as pets, vehicles, or slaughter for private consumption
Prime Minister Gordon Brown is chairing the meeting - an indication of how seriously the government is taking this latest outbreak.
Earlier, the Environment Secretary Hilary Benn said there was "absolutely no truth" in Conservative leader David Cameron's suggestion that the chief veterinary officer had been pressured to declare Britain clear of the disease too soon for economic reasons.
He said immunisation of livestock was being considered.
"We have put arrangements in place to be able to vaccinate if we thought that was the right thing to do," he said.
"The puzzling thing is it's more than 30 days since the last confirmed outbreak. The incubation period for foot and mouth is 2 to 14 days."
Liberal Democrat environment spokesman Chris Huhne said it was crucial to identify the strain quickly to see whether biosecurity at the labs had "failed again".
Experts at the Institute of Animal Health (IAH) in Pirbright, Surrey - the same laboratory at the centre of an investigation into the earlier outbreak - are now working to identify the strain of the virus and to confirm any connection.
Confirmation that it is the same strain as the Pirbright virus could come later today.
Professor Brian Spratt, of Imperial College London, produced a report into the biosecurity leak at Pirbright which led to last month's outbreak.
He also said the Egham case was likely to involve the same strain of virus.
"It's very concerning and very depressing because I think that Defra put in place all the right measures and a week and a half ago they seemed to have it completely under control and everybody was congratulating Defra on a fantastic job of controlling this outbreak," he said.
"Now it all seems to have gone completely wrong and we've got to find out what the reason for it is."
The movement restrictions came into force while livestock sales were taking place around the country.
WHY IS THE VIRUS BACK?
The foot-and-mouth virus is less symptomatic in sheep so an infection from the first outbreak could have been overlooked
The virus can survive in soil for up to 28 days
It can also survive in water for up to 50 days
Foot-and-mouth can also linger in hay and straw for up to 20 weeks
It could be a new strain of the virus, but experts say this is unlikely
Defra said a movement ban on cattle, sheep, pigs and other ruminants has been imposed in England, and parallel arrangements were being made by the Scottish and Welsh administrations.
There is one exception -- a sheep market in the Scottish Highlands is being allowed to go ahead.
The market, in Dingwall, was given an exemption by the Scottish government for welfare reasons. It's thought to be the only livestock market in the UK.
Scotland's chief vet has confirmed that a sheep from Perthshire which was due to be slaughtered in England is being tested for foot-and-mouth.
Charles Milnes said the animal was part of a batch of 131 sheep dispatched to the South of England.
Officials are preparing to carry out tests at the Perthshire farm where the animal originated from.
Mr Milnes stressed that although the animal's symptoms were a concern, tests were still at an early stage
Meanwhile, vets in Norfolk are also investigating a sick animal as a precautionary measure.
Government sources said a temporary control zone has been established around the unidentified location in Norfolk. It's not yet known whether the incident is foot-and-mouth related.
The European Commission has reversed a decision made on Tuesday to lift a ban on meat and livestock exports from Britain to the other UK member states.
It means all live animals susceptible to foot and mouth disease cannot be exported from Great Britain, nor can their products.
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