"Positive test results for foot and mouth disease (FMD) have now been confirmed at the site where it was decided that cattle should be slaughtered on suspicion earlier today," the Department for Environment, Food and Rural Affairs (Defra) said in a statement.
It said the site of the latest case was within an existing protection zone surrounding a premises near Egham in the southern English county of Surrey.
All seven confirmed cases since the outbreak began on August 3 -- the first since a devastating epidemic in 2001 -- have been in Surrey.
Defra said it "remains essential for animal keepers to practice the highest standards of biosecurity, remain vigilant for disease and report any suspicions immediately. Livestock owners should examine their livestock twice a day."
Earlier, Defra said tests for foot-and-mouth on a farm in the neighbouring county of Hampshire had proved negative.
However, a new three-kilometre (1.2-mile) temporary control zone has been set up around a second farm in the county.
"This is a precautionary measure following a veterinary assessment of clinical signs. Laboratory tests are ongoing," a spokeswoman said.
Health officials declared Britain free of foot-and-mouth disease on September 7, after the initial outbreak was believed contained. But just days later, another case was discovered close to the site of the first outbreak.
The first five cases were of the same strain of the virus, which is thought to have leaked from a nearby government-run animal disease testing laboratory and privately-run vaccine facility.
Since August 3, about 1,800 animals have been killed.
There was more bad news for British farmers at the weekend with the confirmation of a case of bluetongue disease, the country's first.
Defra said Monday that a second cow on the rare breeds farm in eastern England had now tested positive for the disease, which causes a swollen head, high fever and the telltale blue tongue. It is not transferable to humans.
"I can confirm a second cow has tested positive for bluetongue and was slaughtered this afternoon on the same farm," said the spokeswoman.
"The evidence remained insufficient to confirm an outbreak."
Britain has become the latest European Union member state to be struck by the serotype 8 bluetongue strain which affects ruminants and for which there is as yet no vaccine.
Spread by midges, the bluetongue strain has moved across Belgium, France, Germany, Luxembourg and the Netherlands and now into Britain.
On Sunday, Defra imposed a temporary surveillance area over several counties in eastern and central England to determine whether bluetongue was spreading.