The European Union 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.
Chief veterinarian Debby Reynolds said 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 Wednesday. The surveillance zone comprises a six-mile radius around the affected farms.
Health and safety experts were working to determine whether the foot-and-mouth disease outbreak came 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 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 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 Department for Environment, Food and Rural Affairs, or DEFRA, said officials were investigating a vegetable plot near one of two infected farms. Newspapers reported that they were looking into the possibility a lab worker had carried the virus to the vegetable patch on boots or clothing.
The disease strain was used in both labs. Merial produced about 10,000 liters of vaccine between July 14 and 25 while the institute conducted a series of small-scale experiments _ using fewer than 10 milliliters in each case, the report said.
Vaccine-maker Merial Animal Health _ the British arm of U.S.-French Merial Ltd. _ said "intensive internal investigations" had found no evidence of breaches.
"To date, we have not been able to establish any evidence that the virus may have been transported out of our center by humans," the company said. "We fully support the investigation and will continue to cooperate and provide complete access to our center to enable these investigations to continue."
The virus was first discovered last week on a farm four miles 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 United States _ 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 during a 2001 foot-and-mouth outbreak, when 7 million animals were killed and incinerated. 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.