'Probable' new foot-and-mouth leak

The virus is believed to have escaped through a leaking valve last week but inspectors have been assured the virus has not been released into the environment, Mr Benn said in a Commons written statement.

The Pirbright site, which also contains the Government's Institute for Animal Health, was the source of the foot-and-mouth outbreak in August.

Mr Benn said the licence to use live viruses for vaccine production was restored to Merial on November 6 following the outbreak in the summer.

New measures were put in place to improve the drainage at the site, with both heat and chemical treatment to destroy live viruses. But Mr Benn told MPs: "On Monday November 19 Merial discovered a shortfall in the quantity of virus recovered from production batches from the previous week.

"On inspection of the vaccine production equipment they identified possible technical problems with the functioning of a valve on a pipe leading from the centrifuge which is used to separate live virus from waste product."

He said operations were immediately stopped and citric acid was used to disinfect any virus which might have leaked from the centrifuge.

An engineer examined the machine on Tuesday morning and investigations focussed on a valve separating the live virus product line from a line providing an outlet for condensation from a steam cleaner.

Mr Benn said safety measures require two operators to certify the valves is shut when the machine is used and that the system is regularly inspected.

"Despite these measures Merial judged that the valve had been leaking, allowing an unintended probable release of live FMD virus into the contained drainage system, which was then pumped to the final chemical treatment facility without being heat treated," Mr Benn said.