A government vet has admitted that his failure to carry out a rigorous inspection at a farm led to the 2001 outbreak of foot-and-mouth disease.
The 2001 foot and mouth outbreak cost the taxpayer £8bn
Jim Dring told of the blunder at a Northumberland farm in official papers.
But the evidence was not put before the Anderson inquiry into the crisis, the Department of Environment, Food and Rural Affairs (Defra) has admitted.
Defra was concerned publication of the comments could prejudice farmer Bobby Waugh's trial, a spokeswoman said.
"This concern centred on putting material into the public domain before it had been dealt with in court," she said.
She added that Defra had already commissioned a note on the origins of the outbreak to submit to the inquiry.
Mr Dring, who is still employed by Defra in Newcastle upon Tyne, said he had not carried out a "rigorous inspection" at the Waughs farm in Heddon-on-the-Wall, Northumberland.
If he had, the outbreak, which cost the taxpayer £8bn, could have been avoided, he admitted in the document, leaked to Farmers Weekly.
Mr Waugh, from Sunderland, was convicted in May of breaching a series of animal health regulations at his farm and was banned from keeping any livestock for 15 years.
In the official document, dated October 5, 2001, Mr Dring said: "Had this inspection been more rigorous... had the licence not been renewed, or renewed only subject to radical revision of the Waughs' patently deficient feeding technique, then this awful 2001 FMD epidemic would never have come about."
Tories and Lib-Dems are now pressing Rural Affairs Secretary Margaret Beckett to make an emergency statement to the Commons.
Shadow Secretary of State for Environment and transport, Rt Hon Theresa may MP said: "The Conservatives have consistently demanded a public inquiry into the foot and mouth crisis which resulted in the slaughter of 10 million animals and cost the UK billions of pounds.
"What seems evident from this admission is that signs of the disease were evident for all to see. There is now an urgent need for an investigation into why such damning evidence was not disclosed earlier.
"Farmers across the country, whose livelihoods were ruined by this crisis, deserve to know the truth."
Defra said the Government had accepted the vast majority of the recommendations by the inquiry conducted by Iain Anderson and there was "no justification for conducting another inquiry".
A National Farmers Union spokeswoman said the NFU would not be considering whether to renew its call for compensation until it had seen the report.