Blood tests on sheep on a Surrey farm suggest exposure to foot-and-mouth disease, officials have said.
Strict precautions have been taken since this recent outbreak began
Preliminary tests were undertaken at the premises within the protection zone near to the latest outbreak.
A spokesman for the Department for the Environment, Food and Rural Affairs (Defra) said the farm's livestock would be slaughtered and further tests made.
Initial tests on pigs culled on another farm proved negative. Full results will be released on Tuesday.
The pigs were on a farm near two premises infected by foot-and-mouth and were culled as a precaution.
Prof Hugh Pennington, emeritus professor of microbiology at the University of Aberdeen, said the possible new outbreak would delay efforts to wipe out the disease.
"It just puts back the day we can declare this is over," he said.
Shadow Defra secretary Peter Ainsworth said the news was "disturbing".
"All of us, especially in the farming community, have been hoping that the outbreak is now over.
"It was caused by negligence by the government and its agencies. We have to hope that the government will now be competent in dealing with the clean-up," he said.
Farmer Derrick Pride, from Elstead, Surrey, had his herd slaughtered in the August outbreak of foot-and-mouth.
"It's utterly devastating," he said.
"I don't know how you can take it really. I'm so disappointed that it has all broken out again."
The restrictions on movement that followed the latest outbreak have affected farmers across the whole of the country.
In the Yorkshire Dales, a two-day annual lamb auction - one of the biggest in Britain - was cancelled due to animal movement restrictions.
The event usually sees £2m exchange hands and is the biggest pay day in the calendar for about 500 local farmers.
However, organisers of the Masham Sheep Fair, in North Yorkshire, have said they will go ahead with their event on the 29 and 30 September, despite foot-and-mouth restrictions meaning sheep cannot be taken there.
On Saturday, the government's chief vet lifted a ban on farmers across most of England from taking livestock to slaughter.
However, livestock can still not be traded or moved for any other reason.
A full ban on any movement remains in place in the surveillance zone around the two infected sites in Surrey.
A 3km (1.8-mile) protection zone has been set up around the farmland, with a 10km (6.2-mile) surveillance zone encircling it.