A foot and mouth outbreak last month was probably caused by leaking drains, flooding and vehicles moving from nearby laboratories, investigators said Friday, without specifying the exact source.
But the Health and Safety Executive (HSE) listed a series of failings at a government-run lab which shares a site with a private vaccine-producing firm at Pirbright in Surrey.
"It is now pretty clear that the outbreak originated at Pirbright but it isn't possible to pinpoint the exact source," said HSE chief executive Geoffrey Podger, presenting a report on the outbreak.
The Pirbright site is shared between the government-run Institute for Animal Health (IAH) and Merial Animal Health, a private company.
Investigators focused on problems with the drainage system at the site, near to a series of outbreaks of the potentially devastating disease at the start of August.
The outbreaks raised the spectre of a repeat of a 2001 crisis, in which up to 10 million animals were culled and which cost the national economy about eight billion pounds.
Leaks were found in a pipe carrying waste from the two laboratories and the virus could have got out due to flooding in July, contaminating the wheels of vehicles on the site which then travelled to nearby farming areas.
Investigators cited disagreements between the IAH and Merial over who was supposed to pay for maintenance of the drainage system.
But the HSE report and another investigation cited failings at the IAH.
Professor Brian Spratt, who conducted an independent assessment, said investigators had "found no problems" with biosecurity at Merial, while the government-run facility was known to be ageing and in need of repair.
In addition "there was some evidence of complacency about safety at IAH," he added.
The criticism of a government-run laboratory risks embarrassing the government of Prime Minister Gordon Brown, who was praised for his swift handling of the crisis after he cut short his holiday just four hours in to deal with the outbreak.
Environment Secretary Hilary Benn accepted the reports' findings but said there was "no excuse" for the outbreak.
"This has been a very difficult time," he said.
"There can be no excuse for the fact that foot and mouth escaped from the Pirbright facility. It should not be possible. It should not have happened even in these extraordinary circumstances, and it must not happen again."
National Farmers' Union chief Peter Kendall blasted the laboratory leaks, which have cost farmers millions of pounds.
"I find it well-nigh incredible and quite indefensible that standards should have been as lax as these reports appear to reveal, given that those concerned were handling some of the most dangerous animal viruses on the planet," he said.
"This outbreak has cost the British livestock industry tens of millions of pounds and it is inevitable that farmers, and many others, will be asking lawyers to consider the case for seeking compensation through the courts for the losses they have suffered.
"We are in discussion with our lawyers about the possibility of seeking legal redress on behalf of our members and will be making a further announcement on that," Kendall said.
But he added: "This should never have been necessary. This was an outbreak that should never have happened."
Chief vet Debby Reynolds meanwhile confirmed that the outbreak had now been completely wiped out -- and announced that clampdown measures imposed are due to be lifted this weekend.
"I am satisfied that foot and mouth has been eradicated from the United Kingdom," she said, adding that the ban on livestock movements and the surveillance zone around the infected farms would be lifted on Saturday.