The recent foot-and-mouth disease outbreak in Surrey was probably caused by leaking drains, heavy rain and building work, a report has found.
Vehicles may have helped spread the virus to the cattle
The Health and Safety Executive said it was not clear which of the two labs at Pirbright, four miles from where the disease was found, were responsible.
Chief vet Debby Reynolds said Surrey was disease-free and the surveillance zone would be lifted on Saturday.
The environment secretary said there was no excuse for the outbreak.
Precise cause 'unknown'
At a news conference in central London, Environment Secretary Hilary Benn said it was not possible to know the exact cause of the outbreak.
Leaking drains may have caused the outbreak, the report found
He said: "What these reports do show is that the most likely explanation for this outbreak is a unique and unhappy combination of circumstances.
"The weaknesses in the drains, the heavy rain and floods, the building work taking place on the site, and the movement of vehicles."
Although he stopped short of placing direct blame, he said: "It should not have happened, even in these extraordinary circumstances and it must not happen again."
The reports, one by the Health and Safety Executive (HSE) and the other by Professor Brian Spratt of Imperial College, include evidence of damage to the pipe with tree roots breaking through and unsealed manhole covers.
HSE Chief Executive Geoffrey Podger said: "We judged it likely that waste water containing the live virus, having entered the drainage pipework, then leaked out and contaminated the surrounding soil."
The report suggests the soil could then have been taken to surrounding farming areas on the wheels of vehicles that visited Pirbright.
The investigators said there had been dispute between Merial, a private pharmaceutical company, and the Institute of Animal Health (IAH) - the two research centres that share the Pirbright site - over who was responsible for maintaining the drains.
Chief veterinary officer, Debby Reynolds, said more than 8,000 samples from animals had been tested over the past two weeks, and all the animals tested were healthy.
However Dr Reynolds said the earliest the UK could achieve international foot-and-mouth disease-free status would be 7 November.
She said she was "satisfied that foot-and-mouth disease has been eradicated from the UK in 2007".
She said the remaining surveillance zone around the affected farms in Surrey would be lifted at midday on Saturday, but the 5km security area around the Pirbright site would remain in place.
All restrictions on export and animal movements have already been removed.
The reports made a number of recommendations that Mr Benn said the government would accept in full.
These include the requirement that Merial and IAH sterilise all waste in a "high containment area;" a revision of what access is given to restricted areas; and the requirement that other laboratories address the issues identified in the reports.
Shadow environment secretary, Peter Ainsworth, told BBC News 24 he was concerned by the findings.
He said: "It's pretty shocking news isn't it that a government-licensed laboratory was responsible for releasing foot-and-mouth into the environment."
The president of the National Farmers Union, Peter Kendall, said it was "indefensible" that such a high-risk environment could have been allowed to get into a state of disrepair.
'Conflict of interest'
He said: "I find it well-nigh incredible and quite indefensible that standards should have been as lax as these."
He said the union was in discussion with lawyers about the possibility of suing for compensation.
Mr Kendall said Defra had done a good job in responding to the outbreak, but said it should never have happened in the first place.
The Liberal Democrat's spokesman on Rural Affairs, Chris Huhne, told BBC News 24 there was a conflict of interest at the Department of Environment, Food and Rural Affairs.
He said: "Defra was warned, and we know this from the reports and the publication of the letters, in July 2004 about the problems with the drainage and they did nothing."
In the outbreak, hundreds of animals were culled and livestock movements were restricted around Britain, at a cost of millions of pounds to farmers.
HOW FOOT-AND-MOUTH MAY HAVE LEAKED OUT OF PIRBRIGHT
Three organisations operate at Pirbright - Institute for Animal Health, Merial Animal Health and Stabilitech
All three work with live virus strain O1BFS that caused foot-and-mouth disease but origin of leaked virus not known
Waste water containing live virus entered drainage pipework
Contaminated waste may have leaked into soil from unsealed manholes and pipes damaged by tree roots
Heavy rainfall in late July may have led to further leaks from drains
Contractors and their vehicles had relatively unrestricted access to Pirbright - a breach of biosecurity
Construction work disturbed soil above drainage system
Lorries probably picked up virus on tyres and underbodies, carried it away from site and past Woolford Farm
Cattle at the farm tested positive for foot-and-mouth on 3 August
Second outbreak at Hunts Hill Farm on 7 August