UK authorities are doing "everything in our power" to eradicate foot-and-mouth disease after it was found at a farm near Guildford, Gordon Brown has said.
Mr Brown said experts would work "night and day" to establish its source.
A UK-wide ban on the movement of all livestock is in place after infected cattle were culled at the Surrey farm.
Defra said it was investigating other possible outbreaks of the disease, and farmers have been asked to go the extra mile with biosecurity and vigilance.
Debby Reynolds, UK Chief Veterinary Officer, said there was a "strong commitment to work with everybody in the countryside concerned".
"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 told a press conference.
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."
Some 60 animals have been culled at the Surrey farm after testing positive.
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 in 2001.
Defra officials have been working at the farm near Guildford
There is also a 10km surveillance zone where nearby animals are monitored, as well as an 8km air exclusion zone around the site.
The prime minister cancelled his holiday in Dorset to return to London for the Cobra meeting.
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.
"We will be doing night and day everything in our power to make sure that what happens happens quickly and happens decisively in a way that can reassure people that everything is being done."
The farm in Surrey has been under restrictions since late on Thursday when symptoms were reported.
Ms Reynolds confirmed the outbreak after samples were taken from the farm.
She said: "It is the absolute priority at the moment to prevent further spread, and piece together information about how it might have got there in the first place."
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
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.
Police have also sealed off Woolfords Lane in Elstead, where locals said a butcher's shop was located.
The outbreak in 2001 led to between 6.5 million and 10 million animals being destroyed and cost as much as £8.5bn.
Officials hope to contain the virus to the immediate locality
The Cabinet Office Minister, Ed Miliband, said there had been a co-ordinated response to the outbreak: "We have had a contingency plan in place following the 2001 outbreak."
Britain has also imposed a voluntary ban on exports of all animals and animal products, the Department for Environment, Food and Rural Affairs said.
The European Commission said it would ban live animal exports from the UK, as well as meat and dairy products from the area affected by the outbreak.
'Stamp it out'
National Farmers' Union president Peter Kendall said they were working with the government to ensure the right steps were taken.
Hugh Pennington, emeritus professor of microbiology at Aberdeen University, said: "We have got to really stop this virus spreading, obviously first of all to stamp it out in the locality where it has been identified and then see whether the virus has got anywhere else."
Tim Bonner, from the Countryside Alliance, said: "Even the words 'foot-and-mouth' will send a chill through the spine of every farmer in the country."
A number of agricultural shows across the UK are going ahead this weekend without cows, sheep and goats, due to foot-and-mouth restrictions.
However shows in Cumbria and Northumberland have been cancelled.
And Woburn Abbey, Bedfordshire, its deer park and Woburn Safari Park are shut.
Whipsnade Zoo, also in the county, has closed its drive-through section.
Defra has set up a helpline in response to the latest outbreak on 08459 335577.
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