A 3km foot-and-mouth temporary control zone has been put in place around premises in East Sussex.
There were outbreaks in Surrey in August and September
It follows a veterinary assessment of suspected signs of the disease in sheep. Tests are in progress on livestock at the site near Rye.
The government had planned to lift the movement ban in low-risk foot-and-mouth areas on 17 October.
The plan also to lift the Surrey foot-and-mouth protection zone was dependent on no further outbreaks.
A Defra spokesman said: "Animal keepers are urged to remain vigilant for disease, check their animals twice a day and report any suspicions immediately."
If confirmed, it would be the first case of foot-and-mouth outside of Surrey since the emergence of the disease in August.
The disease has been identified at eight farms in Surrey since 3 August.
The disease's risk area covers Buckinghamshire, Oxfordshire, Hertfordshire, Berkshire, Surrey, Hampshire and West Sussex, according to Defra.
England's farming community is also reeling from bluetongue disease, although restrictions put in place to prevent its spread were eased on Monday despite the virus spreading to sheep.
The new foot-and-mouth scare comes days after the EU lifted a ban on meat imports from Scotland, Wales and parts of England following the two outbreaks.
All farms outside of an area which takes in south-east England and the Home Counties - deemed a "foot-and-mouth risk area" - and outside of bluetongue control areas - taking in Suffolk, parts of Norfolk, Essex and Cambridgeshire - are now allowed to export meat to the EU.
The National Farmers' Union estimates the farming sector has lost more than £100m since the first foot-and-mouth outbreak in August.