A second outbreak of foot-and- mouth disease is suspected within the 3km protection zone established last Friday around the farm in Surrey where the first case was discovered.
A new herd of cattle has been identified with clinical signs of the disease by Institute for Animal Health staff as part of their surveillance activity within the protection zone.
Chief veterinary officer for England and Wales Debby Reynolds ordered that the herd be culled as soon as practicable.
Dr Reynolds said: "The intensive work of Animal Health has meant that we have been able to rapidly identify this suspect case and take appropriate action swiftly. I continue to urge all animal keepers to be vigilant for signs of disease and practise strict biosecurity."
Samples from the second herd have been sent to a laboratory for testing to confirm the disease.
Dr Reynolds said 50 cattle were involved in the latest suspected case. "The cattle are being slaughtered on suspicion of foot-and-mouth disease. That is beginning tonight," she said.
Dr Reynolds said they were considering whether recent flooding was behind the Surrey outbreak. She said: "The disease can be waterborne, it can also be through the air or some other direct contact. We shouldn't pre-judge the result of the investigation."
She said that on the farm where the first outbreak was confirmed, experts were interested in an area where flood water had stood.
Cuts on the mouths of the infected cattle suggested that they caught the disease some time between July 18th and July 22nd. It is believed there was a flood on the farm on July 20th.
"The investigation on the farm is taking all factors into account, including flooding, movements, everything that's related to the events in the lead-up period to the report of suspicion being made," Dr Reynolds said.
"There is an interest in a particular area of the farm where there was some flood water. That's one of the features that's being examined."
Prof Hugh Pennington, a leading British microbiologist, said that if flood water was to blame for the outbreak it would be most likely to have stemmed from a failure in treating effluent at the Pirbright animal health laboratory site, three miles from the farm.
"If you had a mass of virus going into a water course which connected Pirbright to the farm and then the cattle drank the water . . . that's a theoretical possibility for transmission. But you would have to explain how the virus got into the water."
The Pirbright site is shared between the Institute for Animal Health, which is a government-funded research laboratory, and pharmaceutical company Merial Animal Health, whose work includes manufacturing vaccines.
Both organisations said yesterday that they had found no evidence of a breach in biosecurity.The EU last night banned all British exports of fresh meat, live animals and untreated milk products.