A leaking pipe was the likely source of this summer’s outbreak of foot-and-mouth disease, a report is expected to conclude tomorrow.

The drainage pipe is thought to have been damaged by a tree root, and raises serious questions about the quality of biosecurity and maintenance of infrastructure at the laboratory, shared by a private firm and a government-funded research organisation, in Pirbright, Surrey.

Once traces of the foot-and-mouth virus escaped from the supposedly secure laboratory unit it is believed to have been spread on the vehicle wheels or feet of people working there, possibly contractors upgrading the facilities.

Ministers at the Department for Environment Food and Rural Affairs (Defra), which with the Health and Safety Executive (HSE) has overall responsibility for the site, are braced for bruising criticism when two reports into the outbreak are made public tomorrow morning.

Farmers have been infuriated that an outbreak of the disease should have been the result of a leak from a government-funded research site and the reports are likely to fuel their anger.

Peter Ainsworth, the Shadow Environment Secretary, said: “While there is much speculation about exactly how this outbreak occurred one fact remains the same – that it is both shocking and unacceptable that this virus could have escaped from a government-licensed laboratory at all. Ever since the 2001 outbreak of foot-and-mouth, farmers have been forced to implement an increasing number of stringent biosecurity measures, but it seems that the Government is unable to ensure the safety of its own premises.”

The laboratory site is shared between Merial, a private firm licensed by Defra to produce foot-and-mouth vaccine, and the Institute for Animal Health (IAH), a government-funded research organisation. The suspect pipe is thought to run from the Merial laboratories to IAH, where any viral material contained within waste fluid should have been destroyed safely.

Five years ago the Pirbright site, three miles from the farm where the disease broke out at the end of July, was castigated in an official report for the rundown and unsatisfactory state of several buildings.

Improvements and refurbishment have taken place since the 2002 report but work was still being carried out when the virus infected farms at Normandy in Surrey and there remain serious questions about the Government’s stewardship.

Among the findings are that there was inadequate monitoring of the movements of 120 construction workers, with a failure to keep official records. Hygiene checks on vehicles moving in and out of the site are also expected to be highlighted by investigators as poor.

The discovery that a drainage system used to carry foot-and-mouth and other viruses leaked before the contents could be disposed of safely raises doubts about the level of investment in the site.

Keith Plumb, of the Instituition of Chemical Engineers, said there would be serious questions about any biosecurity system that allowed viral material to be passed through underground drainage pipes from the Merial part of the site to the IAH facilities.

“What does come as a surprise is that Merial were apparently allowed to transfer untreated effluent from their site to the IAH site,” he said. “A virulent and persistent virus like foot-and-mouth should be treated within the same building as it was being used and not transferred anywhere else on the same site, let alone transferred to another site. Transferring dangerous liquid underground is not a good idea since it is difficult to detect leaks.”

Traces of viral material are understood to have been found in the effluent pipe and there is concern that the tree root damage had gone unnoticed by safety inspectors before the outbreak took place.

Two reports are due to be made public tomorrow, one by the HSE and the second by Professor Brian Spratt, of Imperial College, London.

The HSE identified the drainage system as a possible source of the outbreak in its preliminary report in early August, less than a week after livestock movement restrictions were introduced after the discovery of foot-and-mouth in a herd of cattle.

Further tests were carried out as the preliminary report was published and the results were expected to be announced within days but were then withheld, raising suspicions that poor maintenance of the effluent system was the source of a leak.

Professor Spratt’s report addresses the wider issues of biosecurity at the laboratories and funding levels.

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