Tests have shown that the latest outbreak of foot-and-mouth disease in Surrey involves the same strain that infected herds last month.
A 10km control zone has been set up around the site
Chief vet Debby Reynolds said the strain found at Egham was "generally" the same as the strain from Pirbright research site, 10 miles away.
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.
The infected cattle in the latest outbreak are on a farm 10 miles from the Pirbright site, and the disease was confirmed only a week after the previous restrictions on cattle movements were lifted.
Surrey County Council said the area at the centre of the outbreak is grazing land attached to Milton Park farm, while the animals on the land are owned by Hardwick Park farm.
A 10km (6.2 mile) control zone has been set up around the area.
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" among his members.
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
Peter Kendall said this latest outbreak was "much worse" than last month's, because it had come at a time when livestock farmers needed to move their animals, and send them to market.
Mr Kendall attended an emergency meeting of politicians, animal health experts and officials - chaired by the prime minister - to plan the government's next response to the outbreak.
Earlier, 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.
"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," he said.
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 on Thursday.
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.
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 European Commission has banned all live animals susceptible to foot and mouth disease from being exported from the UK.
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