She said about 50 cattle from the second herd were slaughtered on Monday night as a precaution although test results are not expected to be completed until Tuesday.
"We have been able to rapidly identify this suspect case and take appropriate action swiftly," she said in a statement released by the department for food, environment and agriculture.
|Investigators are looking at whether flooding |
may have spread the virus [EPA]
"I continue to urge all animal keepers to be vigilant for signs of disease and practise strict biosecurity."
Gordon Brown, the British prime minister, said a major national effort was under way to halt the infection.
"We are desperately trying at the earliest opportunity to contain and control this disease and then to eradicate it," he said on a visit to a disease control centre close to the affected area.
The foot and mouth virus affects cloven-hoofed animals and is airborne, making it highly contagious.
This is the first outbreak in Britain since 2001, when more than six million animals were incinerated and the cost to agriculture and the rural tourism industry was estimated at £8.5bn ($17bn).
Tracing the source
Biosecurity experts are focusing on two laboratories as the possible source of the outbreak.
The government-run Institute for Animal Health and a facility operated by Merial Animal Health, a US-French company, both handle foot and mouth virus samples.
Both have stocks of the uncommon strain of foot and mouth that was found to have infected the first herd of cattle.
Merial made a batch of vaccines containing that strain of the virus last month.
Both laboratories have since issued statements denying there was any breach of their biosecurity measures.
Government inspectors are expected to issue a report of their findings on Tuesday after conducting a search of both laboratories.
Investigators are also looking into the possibility that flooding had helped the spread of the virus.
A nationwide ban on all livestock movement is expected to last for at least several weeks, raising concerns of a deep economic impact on the meat farming industry that is worth more than $1bn a year.
The European Union has endorsed Britain's decision to ban meat and dairy exports.