A new case of foot-and-mouth disease has been confirmed in Surrey, the government has said.
Defra officials have been working at the farm
Chief vet Dr Debby Reynolds confirmed the outbreak near Milton Park Farm, near Egham. A 10km control zone has been set up around the site.
Cattle and pigs on an adjacent farm are also being slaughtered "on suspicion" of infection, Defra said later.
The European Union has reimposed a ban on UK meat and livestock exports, and a national movement ban is in place.
Prime Minister Gordon Brown said the government had acted swiftly, and the outbreak was under control.
Meanwhile, vets in Norfolk are investigating a sick animal as a precautionary measure.
Government sources said a temporary control zone has been established around the unidentified location in Norfolk.
A sheep which was tested in Scotland for the virus was found to be clear of foot-and-mouth.
The prime minister said the government's first priority was to "contain, control and eradicate" the disease.
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
Mr Brown said he sympathised with livestock farmers but that they would understand the need for a swift response.
"At all times we will be absolutely vigilant - both in looking for the root causes and controlling and eradicating this disease," he said.
He rejected criticisms that the government responses to this outbreak of foot-and-mouth had been too late.
The movement restrictions came into force while livestock sales were taking place around the country.
About 1,300 cattle are stranded in Carlisle, but farmers in Bakewell, Derbyshire, have been granted a temporary licence to move at least 8,000 sheep.
The latest outbreak in Surrey comes just days after the government declared the county to be free of foot-and-mouth.
The farm is about 10 miles from an animal research site in Pirbright, which was the centre of an outbreak in August.
The surveillance zone around the original outbreak was lifted only at midday on Saturday.
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.
Dr Reynolds said up to 300 cattle were being culled on the farm, and a "small number of pigs". The farm is made up of a number of different parcels of land.
Protection zones of 3km (2-miles) have been set up around each farm holding, and a 10km (6.2-miles) surveillance zone imposed.
The M25 motorway cuts through the surveillance zone, which stretches from Slough in the north, to Guildford in the south.
Peter Kendall, president of the National Farmers Union (NFU), said it was a "hammer-blow" to the industry.
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
"Farmers must be extremely vigilant and biosecurity is of paramount importance," he said.
He added that the outbreak could not have come at a worse time, as tens of thousands of stock were due to be moving from upland to low-land farm areas in the next few weeks.
Agriculture ministry 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.
Dr Reynolds urged farmers to remain vigilant and report any suspicions.
Professor Brian Spratt, from Imperial College London, produced a report into the biosecurity leak at Pirbright which led to last month's outbreak.
He 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."
Police have been sealing off roads near the farm
Environment Secretary Hilary Benn talked of his "determination" to contain and eradicate the outbreak.
Conservative leader David Cameron said the outbreak meant there was a real question about government competence.
He said: "Now we need to answer questions about why it was said last week that Surrey was free of foot-and-mouth. Was that the right decision? Was any pressure put on the Chief Vet in order to say that? We need to know that because farmers want these questions answered."
A spokeswoman for Defra denied there was any pressure put on the Chief Vet to lift the ban, saying the suggestion was "nonsense".
Liberal Democrat environment spokesman Chris Huhne said: "It is crucial to quickly identify whether this strain of the virus was being worked on at Pirbright or matches the previous outbreak, and whether biosecurity at the labs has failed again."
Meanwhile, 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.
In a statement the Commission said: "A movement ban on all ruminants and pigs has again been imposed across Great Britain."
It means all live animals susceptible to foot and mouth disease cannot be exported from Great Britain, nor can their products.
Restrictions have been put in place at the farm in Surrey
Earlier in the summer, two farms tested positive for the disease.
Foot-and-mouth was confirmed in a herd of cattle at Woolford Farm in Surrey on 3 August.
A second case, at a farm nearby, was confirmed on 7 August. The all-clear was later given.
A report into the previous outbreak found it was probably caused by leaking drains, heavy rain and building work at the Pirbright site, four miles from where the disease was originally found.
But the Health and Safety Executive report said it was not possible to pinpoint which of the two labs which share the site - Merial, a private pharmaceutical company, and the Institute of Animal Health (IAH) - had been the exact origin of the virus.
A UK-wide movement ban on live animals put in place after that outbreak had been lifted, although a 5km (3 mile) biosecurity zone around the Pirbright site is still in place, according to the Defra website.