Initial blood tests on clinically healthy sheep on a farm within the protection zone set up around last Wednesday's outbreak near Egham, Surrey, suggest exposure to the virus, Defra said.
“These animals were identified as part of the surveillance work that is being carried out in the area,” a spokesman added.
“All animals on the premises are being slaughtered on suspicion. Further laboratory tests are ongoing.”
If the case is confirmed, it would be the third since the virus resurfaced on a farm near Egham last Wednesday.
The Egham outbreak came just days after the government declared the UK to be foot and mouth disease free following an initial outbreak at two sites in Surrey last month.