"No admittance" signs were appearing at farm gates across Oxfordshire today in a bid to curb an outbreak of foot and mouth disease.

The disease has been traced to a farm at Great Horwood, about ten miles from the Oxfordshire-Buckinghamshire border.

Clear message: David Orpwood

A five-mile animal movement exclusion zone has been set up around the farm, prohibiting the movement of horses, pigs, sheep and cattle.

But the public has now been warned to stay away from all livestock farms in the Oxfordshire.

Farmers such as David Orpwood, of Watlington, have already posted Ministry of Agriculture notices telling the public to stay away.

Mr Orpwood, the chairman of Oxfordshire, Berkshire and Buckinghamshire NFU, said: "Farmers should preclude anyone from visiting their farms unless it is absolutely necessary."

He believed the export ban would initially hit sheep farmers who relied most heavily on exports for financial survival at this time of year.

The Ministry of Agriculture has told farmers that anyone working on a farm should change their clothes on leaving the farm.

Yesterday, livestock farmers were ensuring all lorries and boots were thoroughly disinfected before entering and leaving farm gates.

Timothy Blanchard, of New House Farm, Southmoor, near Abingdon, feared the first outbreak of the disease for 20 years would have a devastating effect on prices, as a worldwide ban was placed on all shipments of cattle and meat.

He said: "The public has to be made aware of this disease. But the disease poses no threat at all to consumers."

Tracing the exact source of the disease is now seen as critical, with farmers like Mr Blanchard believing the most likely cause to be imported animals.

Farmers now face an agonizing wait to find out whether the disease has been contained by the measures already introduced.

One theory is that the pigs became infected in transit from Buckinghamshire to the abattoir in Essex. If the animals were infected at the abattoir, it would put the Buckinghamshire farm in the clear, greatly reducing the risk of the outbreak spreading to Oxfordshire.

NFU president Ben Gill said that it was more crucial than ever that farmers and everyone in the countryside continued to be vigilant.

A third case was confirmed yesterday in cattle on a farm in Great Warley, near Brentwood, in Essex. There is an exclusion zone already in place around the farm, which is close to the site of the original outbreak at an abattoir.