The UK has told the European Commission (EC) it wants to classify Great Britain as a high risk zone, from which meat and animal products cannot exported. It will confirm its decision this afternoon, formalising the ban.
Health and safety inspectors are testing a laboratory complex identified as a possible source of the outbreak. Pirbright research centre is home to Merial, the largest specialist animal health company in the world. The Institute of Animal Health (IAH) is also based on the site. Both facilities are licensed to deal with live FMD strains and are three miles away from Woolfords Farm, where the outbreak was confirmed by Defra on 3 August.
The strain of disease found is identical to that used for vaccines and testing at a Pirbright research site.
Gordon Brown said the efforts were to "contain, control and then eradicate this disease". He also said the disease's "transmission mechanism" had still to be discovered.
"I'm determined that we do everything to ensure that the biosecurity that we want to see is properly in place and we can be assured of that," he said.
Farmer Derrick Pride, who runs Woolfords farm in Elstead, Surrey with his wife Sheila and son Roger, told the Guardian the outbreak was "a real shock. I'm devastated," he said.
Defra said around 120 cows were culled in response to the outbreak. Susceptible animals on a farm next to the affected premises had been slaughtered as a precaution because of "potentially dangerous contact".
Environment Secretary Hilary Benn has urged people to remain vigilant as the source has not been confirmed.
Following the arrival of the inspectors at the site, Merial's managing director David Biland said, "our initial investigation shows no breach of our procedures".
Biland stressed that the company's Pirbright centre had produced millions of vaccine doses in the past 15 years without any problems.
"It is too early in the investigation for anyone to determine the source of the outbreak," he said.
The director of the government's IAH, Professor Martin Shirley, said there had been limited use of the strain at the institute within the past four weeks, but insisted there had been "no breaches of our procedures".
He said that the facilities at Pirbright were being redeveloped following a report made in 2002, as a result of the foot-and-mouth outbreak the previous year, which had criticisms of the institute.
The strain of the disease identified at Woolfords farm was also used in a batch of vaccine manufactured on 16 July by Merial. When the strain was identified, Merial voluntarily halted vaccine production as a precaution.
Benn said earlier that safety inspectors would first examine the Merial part of the site, "because we know that vaccines were being produced last month using the particular strain". As well as the health & safety inspection, an urgent review of biosecurity would be carried out at the site, he added. Staff are also expected to be questioned on management procedures, particularly in relation to biosecurity issues.
Benn told BBC News 24 Sunday the link to the Pirbright site was a "promising lead", but he added: "We don't know for sure, and therefore it's very important that people continue to be vigilant."
Defra has increased the size of the protection and surveillance zones covering farms in the area to 10km.
The strain of foot-and-mouth identified is not one normally found in animals but is used in vaccine production and in diagnostic laboratories. In a statement, Defra said: "The present indications are that this strain is a 01 BFS67-like virus, isolated in the 1967 foot-and-mouth disease outbreak in Great Britain."
BBC science correspondent David Shukman said that if the virus did escape from the Pirbright site, the question to ask was how. He said: "Experts speculate that either it escaped through the ventilation or possibly an employee carried it out accidentally on a boot or clothing."
The review of biosecurity measures at Pirbright will be led by Professor Brian Spratt of Imperial College London, who will report back to Benn.
A ban on the movement of all livestock is in place in England, Scotland and Wales. Northern Ireland has imposed a ban on all cattle, sheep and pigs from Britain, but there are currently no restrictions on the movement of livestock within NI and across the border.
David Cameron said farmers are angry and the government may be responsible: "I think there are very serious questions for the government to answer," he said.
"If it turns out that the virus was released either from the Institute for Animal Health in Pirbright or from the next-door lab at Merial - which, by the way, is inspected and licensed by the Government - it will be astonishing news, because the organisations responsible for stopping things like foot and mouth will effectively be responsible for starting it.
"I think the Government will have some serious questions to answer about the report which came out in 2002 that said the facilities were shabby and not up to standard."
"Farmers up and down the country are going to be quite angry because they have done masses to improve their own biosecurity and they are all suffering at the moment, apparently because of mistakes made at a laboratory which is meant to stop foot and mouth.
"While the movement ban is absolutely right... we shouldn't forget that the ban on moving all animals around the country is not pain-free.
"It means abattoirs are closed for business but they still have to pay their staff. It means that markets and auctions are completely out of business while the ban continues. Many farmers who need to move stock can't do that.
"Farmers will be suffering from the movement ban. They will support it - they know it is right - but if they are suffering because others have made mistakes, they have every right to be quite angry."
The outbreak in 2001 led to between 6.5m and 10m animals being destroyed and cost as much as £8.5bn.
Defra has set up a helpline in response to the latest outbreak on 08459 335577.
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