A further case of foot and mouth disease has been confirmed at a farm in Surrey - 30 miles from the Pirbright site linked to the outbreak last month.
Preliminary tests have pointed to the virus and a nationwide ban on the movement of animals has been imposed.
Defra has set up a ten kilometre exclusion zone around Milton Park Farm near Egham in Surrey in an attempt to prevent the virus spreading.
A pre-emptive slaughter was ordered soon after the virus was first suspected this morning.
Chief veterinary officer Debbie Reynolds said: "containment and eradication of foot-and-mouth is our top priority".
Gordon Brown has already spoken with Dr Reynolds and Defra secretary Hilary Benn. He chaired a meeting of the government's emergency response committee at 3pm today.
The National Farmers Union said it was "extremely concerned" at the development, which comes a week after Defra declared foot and mouth had been eradicated in the UK.
The EU has suspended plans to lift the export ban on livestock products, imposed after the original outbreak.
Lib Dem Defra spokesman Chris Huhne said earlier: "If this incident is confirmed as foot-and-mouth, the farming community will be rightly angry and depressed that the likely failings of biosecurity have continued.
Shadow defra secretary Peter Ainsworth said: "This latest suspected outbreak is another bitter blow to the farming community which has just allowed itself to breathe a sigh of relief after the last outbreak."