Experts are investigating whether the latest outbreak of foot and mouth disease in Surrey could have been spread deliberately as an act of sabotage.
"Deliberate human activity" is one of three possibilities being taken seriously by vets and officials at the Department for Environment, Food and Rural Affairs (Defra).
The other leading options are accidental human activity and animal-to-animal transmission, possibly through sheep or deer. It emerged yesterday that the Egham Royal Show, a major agricultural event, took place two weeks earlier in the area where the new outbreak occurred.
A fourth option, transmission through the environment via water or hay, has not been ruled out but is regarded in Whitehall as a much less likely cause.
If any evidence of sabotage were to be found, it could force the Government to reopen the inquiry into the first outbreak of the disease last month. Although a leaking drainage pipe at the Pirbright laboratory complex in Surrey was blamed, officials pointed out that the inquiry report said this was the most likely cause of the problem but was not definitive.
Yesterday Defra confirmed that the new outbreak was likely to have been caused by the same strain of the virus that infected herds last month.
Downing Street strongly denied suggestions by the Tory leader David Cameron that political pressure may have been put on Debby Reynolds, the Government's chief veterinary officer, to declare the first outbreak had been eliminated, which she did last Friday.
Asked if she felt under pressure to lift the bans on animal movements, Ms Reynolds told the BBC: "Absolutely not. It had been forecast from the start that it would be 30 days from cleansing and disinfection of the last Infected Premises at a minimum and when all the work was finished."
Farming industry sources dismissed Mr Cameron's claim as "inappropriate and untimely."
Peter Kendall, the president of the National Farmers' Union, said he was well aware of the "in-depth analysis" taken before the previous restrictions were lifted but added: "If the science turns out that things could have been done differently, we will have to learn from that."
He said this latest outbreak was "much worse" than last month's, because it came at a time when livestock farmers needed to move their animals, and send them to market.
Mr Kendall, who attended an emergency meeting of politicians, animal health experts and officials chaired by Gordon Brown, to plan the response, said there was a " real state of despair" among his members.
Experts at the Government-run Institute for Animal Health in Pirbright, which is 10 miles from the scene of the latest outbreak, were working to identify the strain of the virus and to confirm any connection.
The area involved is grazing land attached to Milton Park Farm. The animals culled belong to Robert Lawrence who owns another farm, Hardwick Park Farm. A second "precautionary" cull of animals took place yesterday at Stroude Farm, owned by Ernest Ward, whose son Steven said 800 pigs and 40 cattle were to be slaughtered.
The Soil Association urged the Government to deploy a strategic vaccination scheme to create a "firebreak" around the latest outbreak to stop its spread. It said the reliance on slaughter alone was failing to contain the disease.
Vets in Norfolk are investigating a sick animal as a precautionary measure at an unidentified location.
Cross-contamination: the theories
The virus may have travelled to Egham from Pirbright on vehicle tyres or on a worker's shoes. The virus can survive in soil for four weeks and in water for seven weeks.
The virus may have been transferred by the Egham farmer from his cattle or an infected field after the ban was lifted.
The virus can be passed between animals, for example by sheep (which show few symptoms), pigs or deer.
Theft of virus samples stolen by eco-terrorists or disgruntled workers.Reuse content