The government ordered three-kilometre (1.8-mile) areas called temporary control zones to be set up around the farms in the neighbouring counties of Kent and Surrey, southeast England, while tests were carried out.
"Both are precautionary measures following inconclusive assessments of clinical symptoms by animal health veterinary staff," the Department for the Environment, Food and Rural Affairs (DEFRA) said in a statement.
The farms are outside the existing exclusion zone, which was put in place after the first case of foot and mouth disease in Britain since 2001 was confirmed near Guildford in Surrey just over 10 days ago.
A second case on an adjacent farm was confirmed shortly after but two more farms where the disease was suspected were later given the all-clear, swelling hopes the outbreak had been successfully contained.
Farmer Stephen Furnival told reporters he had raised the alarm Tuesday at his property in Kent after noticing "unusual lesions" on the muzzles of his calves.
"I felt it was prudent to notify the authorities immediately. Samples will be taken and sent away for testing. All we can do now is wait and get the results, which we sincerely hope will be negative."
Chris Older, from the National Farmers' Union in Kent, said: "This is an example of a farmer doing exactly the right thing -- keeping a careful eye on his livestock and calling in the experts as soon as he noticed something was wrong."
Britain's chief vet Debby Reynolds said containing and eradicating foot and mouth was a priority at what was now a key time in the battle against the virus.
"At this stage the disease has not been confirmed (at the new farms), laboratory results will follow. We are in a crucial week for vigilance and planning next steps," she said.
The government imposed an immediate ban on the movement of cattle, pigs and sheep as well as no-go areas around the affected farms after the first cases were discovered on August 3.
DEFRA said in its statement Tuesday that the national ban would remain in place and that the new farms with control zones would not be allowed to obtain licenses to move dead animals or live animals for slaughter.
Nearly 600 animals on three farms have already been slaughtered, some as a precaution due to suspected "dangerous contact" with infected beasts.
Public health officials are investigating whether the virus may have leaked from an animal disease research centre near the affected farms because the strain was identical to that used there recently.
The site, at Pirbright, houses the government-run Institute for Animal Health and the vaccine producer Merial Animal Health Limited, which is jointly run by US drugs company Merck and Co. Inc. and France's Sanofi-Aventis SA.
Both laboratories have denied any breach in their biosecurity procedures.
The European Union is set to hold a meeting of its veterinary expert group on August 23 to consider the possibility of easing its export ban on British meat and livestock, currently imposed till at least August 25.
British farmers fear a repeat of the costly 2001 foot and mouth outbreak, which also led to an EU export ban and cost the national economy about eight billion pounds while devastating the agriculture sector.
Between 6.5 million and 10 million animals were slaughtered and later burnt on huge funeral pyres.
The government on Tuesday announced a one-million-pound (1.5-million-euro, 2.0-million-dollar) aid package to boost rural tourism in the wake of the recent outbreaks as well as floods in England last month and in June.