Experts searching for the source of the foot-and-mouth outbreak are looking into whether flooding may be to blame.
Chief veterinary officer Debby Reynolds said flooding was one possible means by which animals may have been infected on a farm near Guildford, Surrey.
Inspectors have yet to confirm whether the nearby Pirbright research site, which stocked the virus, is the source.
Earlier, the government said no new cases of the disease had been reported in the previous 24 hours.
Ms Reynolds said no decision had been made on vaccines for livestock, but 300,000 doses had been ordered from Merial - the vaccine manufacturer at Pirbright being investigated as a possible source of the outbreak.
When asked how much Merial would be paid for producing the vaccine, Ms Reynolds said: "I cannot give you the financial implications of that. Merial is the supplier of the UK vaccine bank."
She said the priority remained containment, adding: "It is important that no animals move in Great Britain."
Two facilities based at the Pirbright complex - the private firm Merial and the government-funded Institute for Animal Health - had been using a strain of the virus, for research and for vaccines.
That strain was then discovered at Wolford farm, four miles away, on Friday.
About 97 cows have been culled on the infected farm and neighbouring premises - most as a precaution.
Roger Pride, whose cattle were all culled, told the BBC: "We are still taking it in really...We can do work but it's just a case of waiting to hear the news."
On a visit to a disease control centre in Reigate earlier, Prime Minister Gordon Brown was asked about farmers' anger that a research site could be the cause of the outbreak.
He urged people to await the results of the inspection - expected on Tuesday - but said "every possible source" was being investigated, with up to 100 different farming establishments tested.
Both Merial and the institute have denied any breach in bio-security - Merial says it has "complete confidence" in its procedures - which are now being independently reviewed.
National Farmers' Union head Richard MacDonald said it would be a "body blow" if the vaccine centre was to blame, but also urged people to wait for the results of the inspection.
BBC science correspondent Pallab Ghosh said a flood on 20 July at Wolford farm - where the disease was confirmed on Friday - was one possible means by which animals could have become infected.
He said an investigation of cows' mouths suggested that they were infected sometime between 18 and 22 July. Computer models suggest it was unlikely to have been carried by the wind, he said.
Ms Reynolds said "all factors, including flooding and movements" were being investigated.
"There is a particular interest in a particular area of the farm where there was some flood water," she added.
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
There have also been questions about the Institute of Animal Health - which has been undergoing a £121m redevelopment, following a critical report in 2002. Last year MPs were told there were concerns about cuts to its funding.
But the former head of the institute, Alex Donaldson, said any buildings where the virus was handled met international standards of containment.
He added: "This is the last thing anyone working at Pirbright would want. People are devoting their scientific activities to controlling and preventing foot and mouth disease, not trying to create outbreaks."
Protection and surveillance zones covering farms in the area have been expanded to 10km (6.2 miles).
But there has been some criticism that footpaths remain open in the area.
Conservative leader David Cameron said: "Of course we want the countryside to be open for business but within the exclusion zone it's very important that people don't actually walk from farm to farm, otherwise there will be a risk of spreading it."
Stephen Bowers, who owns a smallholding on the edge of the exclusion zone, also said bridleways were open and wild deer were freely moving across the area.
Two sites on the Pirbright complex are being investigated
But Ms Reynolds said: "Only on the infected premises, and based on veterinary advice, do we always shut footpaths."
She said "some parks, safari parks and other wildlife centres" had closed paths as a precaution, but Defra was not advising that such action was necessary.
The government has banned the movement of all livestock in England, Scotland and Wales.
Meanwhile, the European Commission has formalised a ban on British exports of meat, milk products and live animals. Northern Ireland, which has imposed a ban on all cattle, sheep and pigs from Britain, has been excluded.
Supermarkets in the UK have said they are not expecting any shortages of meat following 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.