Britain has relaxed a nationwide ban on moving livestock after authorities isolated the foot-and-mouth virus to a region near a government laboratory and private company that developed vaccines for the disease.
The EU maintained a ban on British meat and dairy exports, saying it would review the decision in two weeks. Britain retained a self-imposed export ban on such products.
Farmers outside a surveillance zone set up around the farms where the outbreaks occurred would be able to send their animals to slaughterhouses as of midnight on Wednesday, chief veterinarian Debby Reynolds said.
The surveillance zone comprises a 10km radius around the affected farms.
Health and safety experts were trying to determine whether the outbreak of foot-and-mouth disease had originated from a high-security government laboratory or from a private pharmaceutical company on the same site -- and whether its spread was accidental or deliberate.
Despite easing the transport ban, authorities ordered the slaughter of livestock on a third farm suspected of having the disease. The farm was next to another where cases of foot-and-mouth were confirmed on Tuesday.
Reynolds said the order was a precaution and tests were under way to determine whether any more animals had been infected. She said the strain found on the second infected farm was identical to that found at the first outbreak -- and that used in the labs.
She said there was a "low, but not negligible" risk of the disease spreading outside the surveillance zone set up around the lab and the affected farms.
The country's health and safety agency said in a report late on Tuesday that there was a "strong probability" the outbreak originated at the Pirbright lab site southwest of London and was spread by human movement.
But the drug company being investigated as a possible source of the outbreak insisted there had been no breach of its biosecurity procedures.
The virus was first discovered last week on a farm 6.5km from the Pirbright lab complex.
The government's Health and Safety Executive said there were various potential routes for "accidental or deliberate transfer of material from the site."
Foot-and-mouth can be carried by wind and on the vehicles and clothes of people who come into contact with infected animals.
China and Mexico on Wednesday joined other countries -- including the US -- in banning imports of British livestock and their products.
Britain has voluntarily suspended exports of livestock, meat and milk products in response to the outbreak.
The outbreak fed fears of a repeat of scenes in 2001, when 7 million animals were killed and incinerated on pyres.
Britain's agriculture and rural tourism industries were devastated.
Foot-and-mouth disease affects cloven-hoofed animals, including cows, sheep, pigs and goats, but does not typically affect humans.