Farmers have been urged to remain vigilant for signs of foot and mouth among their livestock, amid speculation that the virus may have leaked from a research laboratory.

Chief Veterinary Officer Debby Reynolds revealed that the strain of virus responsible for an outbreak on a Surrey farm was the same as one produced at the Pirbright labs a few miles away.

But Environment Secretary Hilary Benn stressed that, while this was a "promising lead", it was too early to say that the site - home both to the official Institute for Animal Health and the commercial pharmaceutical company Merial Animal Health - was definitely the source of the infection.

An investigation is underway by experts from the Health and Safety Executive into the Pirbright site and Mr Benn has commissioned a review of biosecurity arrangements led by Professor Brian Spratt of Imperial College, London University.

Bans on the movement of cows, sheep and pigs and the export of cloven-hoofed animals and animal products remain in force.

Mr Benn said: "The important thing to bear in mind is that this is a promising lead, but we don't know for sure and therefore it is very, very important that people continue to be vigilant.

"All the measures that we put in place on Friday evening remain in place."

Since foot and mouth was first identified on the farm in the village of Normandy, near Guildford another four farmers have reported concerns about their livestock.

All four have been checked by vets and given the all-clear.

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