A fresh case of foot-and-mouth disease is suspected in Surrey, the chief veterinary officer has confirmed.
Protection zones surround the affected sites
Cattle, within the 3km protection zone set up around the farm where the first outbreak occurred, are being culled as a precaution.
Chief veterinary officer Debby Reynolds said they had detected "suspect" signs and the cows were being culled to "minimise any chance of spread".
Test results, due on Tuesday, will establish if it is another outbreak.
The BBC understands the new foot-and-mouth cull will affect between 50 and 100 cattle.
Dr Reynolds said the second potential outbreak site was a "small number of kilometres" from Wolford farm, near Guildford, the first infected premises.
Vets identified clinical signs of the disease in a herd on one of their regular visits to farms in the protection zone.
Samples are being analysed for official confirmation.
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 practice strict biosecurity."
Environment Secretary Hilary Benn said the latest discovery showed "the benefits of the firm measures that we have taken to deal with this outbreak and stop its spread".
"It was the surveillance team going round that noticed what could be the first clinical signs of foot and mouth," he told BBC Two's Newsnight programme.
Microbiologist Professor Hugh Pennington said people should not read too much into the second cull because the government would now be acting on even the "slightest whiff" of further infection.
Mr Pennington said it was vital to establish the source of the outbreak in order to gauge the extent of the problem.
"Until it is decided how the virus got to the farm, there remains the possibility that - if it was carried on the wind - it could have infected contiguous areas."
Earlier, Dr Reynolds said flooding was one possible means by which the animals may have been infected.
It has also emerged that the cattle at Wolford farm began displaying signs of illness five days before the authorities were notified.
A government report states the illness was first seen in the cattle on 29 July but restrictions were not imposed on the premises until late Thursday, 2 August.
Inspectors have yet to confirm whether the nearby Pirbright research site, which stocked the virus, is the source.
'Every possible source'
Dr Reynolds said no decision had been made on vaccines for livestock, but 300,000 doses had been ordered from private firm Merial - the vaccine manufacturer at Pirbright being investigated as a possible source of the outbreak.
Mr Benn, defending that decision, said: "In order to consider [vaccination] as an option, we've got to have the vaccine ready.
"It's the best place we can go if we take the decision that we want to do it."
He would not comment on the suggestion that Merial should be stripped of subsequent profits if it was found to be responsible.
Two facilities based at the Pirbright complex - Merial and the government-funded Institute for Animal Health - had been using a strain of the virus, for research and for vaccines.
That strain was then discovered at Wolford farm, four miles away.
On a visit to a disease control centre in Reigate, Prime Minister Gordon Brown was asked about farmers' anger that a research site could be the cause of the outbreak.
He urged people to await the results of the inspection - expected on Tuesday - but said "every possible source" was being investigated, with up to 100 different farming establishments tested.
Both Merial and the Institute have denied any breach in bio-security - Merial says it has "complete confidence" in its procedures - which are now being independently reviewed.
National Farmers' Union (NFU) head Richard MacDonald said it would be a "body blow" if the vaccine centre was to blame, but also urged people to wait for the results of the inspection.
Officials set up a 3km (1.8m) protection zone and a surveillance zone with a minimum radius of 10km (6.2m) around the first site.
The government has banned the movement of all livestock in England, Scotland and Wales.
Meanwhile, the European Commission has formalised a ban on British exports of meat, milk products and live animals. Northern Ireland, which has imposed a ban on all cattle, sheep and pigs from Britain, has been excluded.
The NFU has estimated the outbreak could cost "tens of millions of pounds", affecting not just farmers but related industries, such as abattoirs and meat packaging plants.
Anthony Gibson, NFU director of communications, said: "If things go wrong and we do get further outbreaks and people in the meat trade move to exploit the situation, we could be looking at costs running into tens of millions of pounds."
Shadow environment secretary Peter Ainsworth said the possibility of a second outbreak reinforced the need for "constant vigilance".
He called for "a more prudent approach to the public access in the vicinity of the infected area".
He added: "Local farmers are understandably concerned that, whilst they cannot move animals at all in the area, people can walk freely through it, increasing the risk of spreading the disease."
Defra has set up a helpline in response to the latest outbreak on 08459 335577.