Health and safety inspectors have arrived at the laboratory complex identified as a possible source of the foot-and-mouth outbreak in Surrey.
The strain of the disease found is identical to that used for vaccines and testing at a Pirbright research site.
Inspectors will first be examining Merial Animal Health, a private pharmaceutical company on the site.
Prime Minister 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.
Ban stays in place
Mr Brown said the inspectors' report would be completed in the next 48 hours - and the ban on the movement of cows, sheep and pigs would remain in place.
The Department for the Environment, Food and Rural Affairs said around 120 cows in all had been 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".
There have so far not been any further outbreaks, but 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".
Mr 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," said Mr Biland.
As well as Merial Animal Health, the Pirbright site houses the Pirbright Laboratory, a research facility of the government's Institute for Animal Health (IAH).
The institute's director, 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 Wolford farm, near Guildford, 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.
Prof Martin Shirley insisted IAH procedures had not been breached
Mr 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 and 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.
Mr 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."
Conservative leader David Cameron said that if the virus was found to have been released from the Pirbright site, then it would be "astonishing news, because the organisations responsible for stopping things like foot-and-mouth will effectively be responsible for starting it".
Defra has increased the size of the protection and surveillance zones covering farms in the area to 10km.
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
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 Mr 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.
Britain has also imposed a voluntary ban on exports of all animals and animal products, Defra said, and 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.
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.
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