Blunders at a government-run laboratory have been blamed for last month's outbreak of foot-and-mouth disease, which cost British farmers more than £30m. A combination of leaking pipes, rows over who should pay for repairs, poor checks on vehicles leaving the site and unexpected flooding was the most likely cause of the virus escaping, two official inquiries concluded in reports published yesterday.

Last night, farmers' leaders threatened legal action against the Department for Environment, Food and Rural Affairs over the mistakes. Peter Kendall, of the National Farmers' Union, said he found it indefensible that standards were so lax, given that "those concerned were handling some of the most dangerous animal viruses on the planet".

Hundreds of animals had to be put down and tough controls were imposed on the movement of livestock after foot-and-mouth was found on two farms near Guildford, Surrey. The virus leaked from of a site at Pirbright, where both the Government's Institute of Animal Health (IAH) and the drugs company Merial conduct research. Yesterday's report by the Health and Safety Executive pinpointed several biosecurity breaches at the IAH, while an independent expert, Professor Brian Spratt, condemned the "complacent" inspection and record-keeping at the plant.

Investigators concluded that the virus probably escaped from damaged drains, where pipes were pierced by tree roots and manhole covers were not sealed properly. The virus is thought to have reached the surface after torrential rain at the end of July. Contaminated mud is believed to have been transported out of the site on the wheels of building company lorries.

Concerns about the broken drains were raised in 2003 but, despite £30m being spent on the site recently, the drains were not replaced because of wrangling between the IAH and Merial over who should pay. Hilary Benn, the Environment Secretary, said: "These reports show the most likely explanation for this outbreak is a unique and unhappy combination of circumstances." He said no one would face disciplinary action but insisted that "lessons would be learnt".

Mr Kendall added: "We are in discussion with lawyers about the possibility of legal redress on behalf of our members."

The blunders

* Leaks from pipes caused by tree roots

* Poorly fitting drain covers

* Pipes not replaced because of cash wrangles

* Drains could not cope with floodwater

* Irregular inspections of site

* No checks on vehicles moving in and out of facility

* Poor communications between IAH and Merial

* Evidence of 'complacency' about safety