The deliberate release of viral material, possibly in an act of sabotage, may have caused the foot-and-mouth disease outbreak, officials said last night.

The Health and Safety Executive (HSE) said in a report ordered by the Prime Minister that “release by human movement [of the FMD virus] must be considered a real possibility”.

Inspectors all but discounted theories that the virus escaped by air or water from the laboratory complex close to where the outbreak started, although they are continuing to investigate the possibility of equipment failure or a security breach.

The HSE concluded in the report — which was sent to Hilary Benn, the Environment Secretary, last night — that there was “a strong probability” that the virus came from the research centre three miles from the first outbreak in a herd of cattle in Surrey.

Mr Benn said that sabotage could not be ruled out. “The truth is we do not know. That is why further investigations need to take place.”

The finding strengthens suspicions raised at the weekend when it was discovered that the strain of virus was one found only in laboratories. Investigators were unable, however, to decide whether the foot-and-mouth contamination escaped from the Institute of Animal Health, the government-funded body that first identified the virus, or Merial, a private company that shares the same site at Pirbright.

Further genetic analysis of the virus is being carried out to determine from which of the laboratory units the virus originated.

Any security breach could have been either deliberate or accidental and it was confirmed last night that government scientists have been inspecting allotments close to the first outbreak which are used by Pirbright workers to grow vegetables.

The report was released yesterday evening more than six hours after the 48-hour deadline requested by Mr Benn. The reasons for the delay were unclear, despite suggestions that “political and presentational” concerns were a factor.

Should investigators find that biosecurity systems or equipment at IAH were in any way to blame for the outbreak, ministers would face allegations that the failure was caused by years of underfunding.

If Merial is identified as the source of the disease outbreak the firm would face the potential of a multimillion-pound class action for damages by farmers whose businesses have been affected. Both Gordon Brown and Mr Benn appeared to point the finger of blame at Merial last night. They both suggested that further investigations would concentrate on equipment at Merial whereas the report that had been made public made no distinction between the company and IAH.

After the publication of the HSE’s report, the Prime Minister said: “The work goes on to isolate, contain, control and eradicate the disease.”

In their preliminary report on the outbreak, the HSE said there were “a number of biosecurity issues” that remained to be addressed at the complex. “There are various potential routes for accidental or deliberate transfer of material from the site,” they wrote. “Release by human movement must also be considered a real possibility. Further investigation . . . is being urgently pursued.”

They regarded as unlikely the chances of the virus being blown from one of the laboratory units and then being carried by wind to fields near the village of Normandy where they infected cattle on two farms.

Flooding from a failure of the effluent systems on the research site could not be discounted, but HSE inspectors said that they had been unable to find evidence of mechanical problems. The chance of water getting from Pirbright to the fields at Normandy was regarded as “negligible due to the distance”.

Geoffrey Podger, chief executive of the HSE, said: “The investigation to date has concentrated on biosecurity issues associated with FMDV strain 01BFS67 as this was the strain associated with the outbreak in farms in the Pirbright vicinity.”

The investigations concentrated on the actions of the IAH and Merial from July 14-25 as the dates identified by Defra when the escape must have taken place. Both research organisations used the viral strain during that time but whereas IAH employed it only in small scale experiments, Merial was involved in creating “large scale production” of 10,000 litres.

The HSE team of six investigators “found no evidence” of working practices, spills or leaks that could have led to a release of the virus.

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