The strain of foot-and-mouth disease found at a Surrey farm has been identified, Defra has said.
The strain in infected cattle is identical to that used for vaccines at an animal disease research site at Pirbright, three miles from the farm.
Defra could not say the laboratory complex was the source but has increased the size of the protection and surveillance zones in the area.
An urgent assessment of biosecurity has begun at the institute.
The strain is not one normally found in animals but is used in vaccine production and in diagnostic laboratories.
In a statement, the Department for Environment, Food and Rural Affairs (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."
The strain was used in a vaccine batch manufactured on 16 July by a private pharmaceutical company Merial Animal Health.
The firm shares the Pirbright site with the government's Institute for Animal Health (IAH), which conducts research into foot-and-mouth and where the strain is also present.
Merial voluntarily halted vaccine production as a precaution.
BBC science correspondent David Shukman said that if the virus did escape from the Pirbright laboratory, the question to ask was how.
He said: "Like the manufacture of any vaccine to defend against a virus, this one used samples of live virus in the production process.
"Experts speculate that either it escaped through the ventilation or possibly an employee carried it out accidentally on a boot or clothing."
Chief Veterinary Officer Debby Reynolds said it was too soon to say anything conclusive about the source of the virus but it was clear which strain was involved.
"That is carried at the Pirbright facility, at Meriel for vaccine production, and indeed at the Institute for Animal Health, on the same premises on the same location for diagnostic purposes," she said.
Microbiology expert Hugh Pennington said that if the source is identified as the Pirbright laboratory it could be welcome news.
He said: "If we know exactly where the virus has come from, and particularly if it's a vaccine type of virus, it's less likely to be a nasty virus.
"We know there isn't uncertainty about the source, so that means there isn't going to be virus in Cumbria or in Scotland, or in Wales from the same source as the virus that has caused this infection in Surrey, it's a localised problem."
An urgent review has been launched into biosecurity measures at Pirbright, led by Professor Brian Spratt of Imperial University, who is due to report to Environment Secretary Hilary Benn and Dr Reynolds.
Defra said that it was too soon to reach any firm conclusions.
Merial said in a statement: "The decision to suspend production has been taken in full consultation with Defra and will enable Defra to carry out a thorough investigation into all possible sources of this outbreak."
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
Following the outbreak on the farm a 3km protection zone was 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.
Dr Reynolds has now ordered a single protection zone to encompass both the infected farm premises and the Pirbright site, with a single 10km radius surveillance zone.
There is also an 8km air exclusion zone around the site.
There has been a cull of one other herd of cattle adjacent to the farm as a precautionary measure but there were no signs of infection in any animals there, Dr Reynolds confirmed.
She appealed to farmers to be vigilant and to check their animals for any sign of foot-and-mouth disease.
Andrew Biggs, of the British Cattle Veterinary Association, said: "It doesn't surprise me greatly. The proximity of this farm to Pirbright was something some of us had noticed."
Paul Temple, of the National Farmers' Union, said he was keeping an open mind over the situation.
A ban on the movement of livestock in England, Scotland and Wales which put in place after foot-and-mouth was confirmed at Wolford farm, near Guildford, on Friday night will continue.
Some 64 cattle have since been culled at the farm after testing positive for the foot-and-mouth.
On Saturday evening Prime Minister Gordon Brown chaired his second Cobra emergency committee meeting of the day on the issue after he cut short his Dorset holiday to return to London.
He is due to chair another meeting of Cobra on Sunday morning.
The outbreak in 2001 led to between 6.5 million and 10 million 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.