Wiltshire farmers' hearts will be in their mouths today as they wait for scientists to find the cause of the latest foot-and-mouth outbreak.
It has yet to be confirmed if a newly-discovered infection on farmland in Egham, Surrey is the same strain which infected herds last month.
But already there is a nationwide movement ban on cattle, sheep, pigs and other susceptible animals and restrictions on livestock markets and shows and the movement of animal carcasses.
Shearing and dipping is also restricted and farms have been urged to increase their biosecurity measures.
The new scare comes just a week after the Environment Secretary declared Britain to be free of the disease and it has shocked the farming industry.
A second cull of animals on land next to the farm was ordered last night on suspicion they too had been infected following animal health inspections after the first outbreak.
Experts at the Institute of Animal Health (IAH) in Pirbright, Surrey, are now working to identify the strain of the virus to see if the two outbreaks are connected.
The Government-funded institute was the focus of the investigation into how the disease was spread to two herds in August and faulty pipework on the site was later blamed.
But this has now been repaired and the IAH has already said there is currently no evidence to say how the latest farm was infected or to suggest any further leaks from Pirbright.
Prime Minister Gordon Brown has pledged that the authorities would do "everything in our power" both to eradicate the disease and to track down the source of the latest outbreak.
The latest incidence of the disease is around 10 miles from the two previous cases near Pirbright, which are thought to have been caused by a leak from laboratories there.
Surrey Council said the area involved was grazing land attached to Milton Park farm and that the animals there were owned by another farm, Hardwick Park farm.
Initial tests on animals on the farmland show they had the virus and the cattle have been culled as a precaution after displaying symptoms of the highly infectious disease.
Protection and surveillance zones are now in place to try to contain it and most of Windsor Great Park, which is inside the 10km surveillance zone, has been closed to the public.
The Department for Environment, Food and Rural Affairs (Defra) said last night that animals on land adjacent to the farm were also to be slaughtered as a "precautionary measure".
The second cull came after animal health inspectors spotted the animals yesterday afternoon as they investigated the first case. It was advised "on suspicion" of infection.
Surrey County Council's trading standards team is to close footpaths and other rights of way within the protection zone although paths will remain open in built-up areas.
The European Union has reversed a decision to lift the ban on British beef exports and imposed emergency measures declaring the whole of Britain a "high risk" area until mid-October.
Farming leaders described the latest case of foot and mouth as a "hammer-blow" to the industry, coming just days after the UK was given the all-clear.
Having declared the country free of the disease last week, Environment Secretary Hilary Benn yesterday described the discovery as "news that nobody wanted to hear".
Chief Veterinary Officer Debby Reynolds said last night: "This is a developing situation. Our objective is to contain and eradicate the disease.
"As with the outbreak last month, we will be seeking to take a staged and risk-based approach to controls. I urge all animal keepers to remain vigilant and follow the most stringent of biosecurity measures."
She had said earlier yesterday that the strain of the virus and its origin had not yet been identified.
But the National Farmers Union (NFU) believes the chances of it being a different strain or source to the one behind the original outbreak are "incredibly small".
President Peter Kendall said: "It is likely it all goes back in some way to the original outbreak near Pirbright."
He said the latest case would have "enormous" ramifications for the whole of the farming community.
"At the weekend the whole industry breathed a collective sigh of relief that we had moved on. This has set us right back."
Liberal Democrat Environment, Food and Rural Affairs 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.
"If so, work should stop until there are fundamental improvements at the site."
Tory MP Philip Hammond, whose constituency includes Egham, said if there is a link with the August outbreak, the first question to ask was why the restrictions on moving livestock were lifted.
He said: "Presumably it was on the basis of a scientific assessment that the incubation period was only a couple of weeks.
"Either the decision to allow movement or the information they have is wrong. Are we looking at something completely new in terms of how the virus behaves?"
"We may need to change the way we approach this problem."
Chief executive of the Countryside Alliance, Simon Hart, said "serious questions" had to be answered.
He said: "Mr Benn must be honest and open about how this outbreak occurred, and he must be brutal in rooting out the cause.
"The onus is on Defra to pinpoint the cause and to get rid of the disease as quickly as possible...The priority for everyone, led by Defra, must be eradicating the disease and getting British farming back to health as quickly as possible."
The NFU said the value of exports, including meat, meat products and dairy, is £1.8 million per day - a sum which will be lost due to restrictions in the wake of the latest outbreak.
A spokesman said last month's outbreak is likely to cost the farming industry between A£50 million and A£80 million.