Officials were last night trying to trace how the latest outbreak of foot-and-mouth occurred, as initial tests indicated the strain of the virus was the same as that which infected herds last month.

The findings came as an investigation ruled out the disease in a sheep from a Perthshire farm, which was due to be slaughtered in England.

Chief Veterinary Officer Debby Reynolds said the strain infecting cattle near Egham, Surrey, was "generally" the same as that which caused the August outbreak near Pirbright, 10 miles away.

The results of a full examination of the strain are expected later.

As culling on farmland near Egham continued last night, Dr Reynolds said an epidemiological investigation was looking at whether the disease had spread through animals, vehicle movements or environmental contamination.

The government denied that that Dr Reynolds had been pressurised into giving the all-clear too early after the previous outbreak, which was blamed on leaking pipework at Pirbright laboratory site.

Surrey was declared free of foot-and-mouth, along with the rest of the UK, last Friday. Days later, the disease re-emerged.

A spokesman for Prime Minister Gordon Brown said: "This was a decision that was made on the basis of scientific evidence. It was a decision made by the Chief Vet and it was a decision supported by many academic experts."

The spokesman said there was "absolutely no truth" in any suggestion that political pressure had been applied to influence Dr Reynolds.

However, Peter Ainsworth, the Shadow Environment Secretary, said: "It comes as no surprise to learn the latest outbreak is the same strain as that of the outbreak in August.

"But it does raise further questions about why the government felt able to declare that foot-and-mouth disease had been eradicated'.

This was clearly a tragic misjudgment."

Speaking after a meeting with Mr Brown and Hilary Benn, the Environment Secretary, National Farmers' Union president Peter Kendall said they shared a "real state of despair" over what has happened. He added: "We are behind the initiative to control, contain and eradicate."

The Scottish Government said yesterday a suspect sheep was not suffering from foot-and-mouth. A spokesman said: "Vets have now completed their investigations into the reported sickness of a sheep at an abattoir in England and its flock that originated from Perthshire. Examinations have eliminated foot-and-mouth."