More farms are being tested for foot-and-mouth disease after an outbreak at a farm near Guildford.
Defra said other possible cases were being investigated after infected cattle were culled at the Surrey farm.
A UK-wide ban on the movement of all livestock is in place, and farmers have been asked to "go the extra mile" on biosecurity and vigilance.
Prime Minister Gordon Brown said experts were working "night and day" to find the source of the disease.
A 3km protection zone has been put in place around the premises, close to the village of Wanborough, to try to halt the spread of the disease which wreaked havoc across the UK in 2001.
There is also a 10km surveillance zone where nearby animals are monitored, as well as an 8km air exclusion zone around the site.
The UK's Chief Veterinary Officer Debby Reynolds told a press conference a "small number" of further potential foot-and-mouth cases were being investigated.
"Some have been reported and found to be negative, others are still under investigation," she said.
An assessment of biosecurity at the Institute for Animal Health in Pirbright was also under way, she added.
The Pirbright institute, where research into foot-and-mouth is conducted, is located not far from the Surrey farm.
Ms Reynolds said "all possible sources" of the disease were being investigated.
"It's important not to rule out any source in our inquiries," she said.
"It is a big blow for it to be back in UK territory, but our objective is to eradicate it and to learn the lessons learned after 2001, to use the lessons learned for a speedy, systematic and scientific response," she said.
Defra also announced it would set up a national disease centre in London, and a local one in Surrey as part of contingency plans.
Richard MacDonald, director general of the National Farmers' Union, said it had been a "devastating 24 hours for the livestock industry".
"We are where we are now and we take that obviously hugely seriously and, given all of that, our priority is to ensure that we get out of this situation as quickly as possible and as effectively as possible."
The prime minister cancelled his holiday in Dorset to return to London for a meeting of the government's emergency Cobra committee. Later, Mr Brown chaired his second Cobra meeting of the day.
Foot-and-mouth is a highly contagious viral disease which affects cattle
Symptoms include fever, lesions in the mouth and lameness
The disease only crosses the species barrier from cattle to human with very great difficulty
The disease in humans is mild, short-lived and requires no medical treatment
Mr Brown said: "I want to do everything in our power immediately to get to the scientific evidence, to look at the source of what has happened, to set up a number of inquiries so that we can actually move very quickly, I mean within hours and days, what has actually happened, and then to eradicate this disease in Britain".
The infected farm in Surrey has been under restrictions since late on Thursday when symptoms were reported.
Some 64 animals were culled at the farm on Saturday, after testing positive for the disease.
Scientific analysis of the virus could be available late on Saturday, but it may take longer depending on the exact strain of the disease involved.
Ms Reynolds has advised farmers across the UK to examine their animals carefully and immediately report anything suspicious.
The outbreak in 2001 led to between 6.5 million and 10 million animals being destroyed and cost as much as £8.5bn.
Investigations into other potential outbreaks are under way
The Cabinet Office Minister, Ed Miliband, said there had been a co-ordinated response to the outbreak.
A contingency plan had been in place since the 2001 outbreak, he said.
Britain has imposed a voluntary ban on exports of all animals with cloven hooves, and animal products, Defra said.
And the European Commission said it would ban live animal exports from the UK, excluding Northern Ireland, as well as meat and dairy products from the area affected by the outbreak.
Conservative leader David Cameron, who has spoken with Mr Brown, said he fully supported a ban on the movement of farm animals.
"Livestock farmers face real difficulties with higher feed prices, the dislocation caused by floods and TB in cattle. We must do everything possible to make sure this is not a further blow to farming."
He also encouraged people who has intended to holiday in the countryside to stick with their plans.
"Farmers and rural communities should know that the whole country is on their side," he said.
Liberal Democrat leader Sir Menzies Campbell also spoke to Mr Brown following the Cobra meeting, his office said.
A number of agricultural shows across the UK are going ahead this weekend without cows, sheep and goats. However shows in Cumbria and Northumberland have been cancelled.
Defra has set up a helpline in response to the latest outbreak on 08459 335577.
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