Ministers are frustrated by the failure of the Health and Safety Executive to assert that a private laboratory at Pirbright, Surrey, was to blame for the leak which led to the outbreak of foot-and-mouth disease.
A letter from the investigators points the finger of suspicion at the private labs of Merial, the United States-based company which runs a vaccine-manufacturing plant next door to the research laboratories of the world-renowned public Institute for Animal Health (IAH).
The letter was sent to the Environment Secretary, Hilary Benn, by Dr Paul Logan, the lead investigator for the Health and Safety Executive, with its interim report on the outbreak. It was seen by ministers as a signal that the public laboratory would be cleared.
"My assessment is that there is no reason to prevent the Institute for Animal Health from operating providing that all the usual biosecurity protocols are followed rigorously," said Dr Logan.
"The situation regarding Merial is less clear-cut, and I would advise that further work be done before any operations involving live pathogens are restarted."
Dr Andrew King, former head of the department of molecular biology at IAH Pirbright, said yesterday that high standards of biosecurity were kept at the public laboratory.
"Leaving the building you have to change and have a shower," he said. "Staff are not permitted to have contact with animals, or visit premises where susceptible animals are kept for a period of three or five days. If they work in the labs where the testing work is done that's three days. If they are in animal houses then it is five days."
Suspicion continued to hang over the public laboratories yesterday. The outcome could be costly for the Government, which is still facing claims for millions of pounds of compensation from farmers across the country for lost business.
Security checks are believed to be under way on the laboratory staff at Pirbright to rule out the possibility that the disease was spread deliberately as an act of bioterrorism.