Food safety watchdogs launched an urgent investigation yesterday into a "serious failure" of anti-BSE measures designed to protect consumers from the human form of the cattle disease.
Meat from about 200 cattle that should have been tested because they were at higher risk of having the disease went into food without checks.
The errors, made over 16 months, are said by the Food Standards Agency (FSA) to pose "minimal risk" but they are deeply embarrassing given that the government is considering whether to relax other controls as the disease declines, in animals at least.
The BSE crisis has lasted more than 17 years, and there has been a sorry history of failures in controls, although regimes have been tougher since the first link between the disease and deaths in young people was made in 1996.
The failures, called "unacceptable" by the FSA chairman, Sir John Krebs, were down to part of the agency itself, the Meat Hygiene Service, responsible for monitoring the slaughter of animals.
Only cattle under 30 months old are allowed to go into food, because most animals that show symptoms of BSE are far older. The last under-30-month case was eight years ago. In addition, the organs and body parts most likely to carry infection have to be removed, a safeguard the agency says removes 99% of the risk of BSE in food.
But so-called casualty animals, which are culled after injury or illness at 24 months or older, have to be tested for BSE before being used for food - a relatively late measure, introduced in 2001 under EU rules.
It is some of these animals that have slipped through the net, although a further 2,800 casualty animals that have been tested have shown no sign of disease.
The agency recommended last year that animals over 30 months might be allowed into food provided they were rigorously tested, but ministers have asked the agency to conduct further risk assessments before reaching a decision.
Recent research has suggested that as many as 3,800 Britons may unknowingly be harbouring variant CJD, BSE's human form, while some scientists believe there may also be more than one form of BSE.
An FSA spokesman said: "Clearly there has been a serious failure in the current testing system that will not inspire confidence and needs to be put right."
"This does highlight the importance of making sure that an effective and robust testing system is in place before any decision on a move to replace the over-30-month rule is implemented."
In a separate statement, Sir John said: "Whilst the risk is minimal, I consider these apparent failures unacceptable. We need to be confident that all measures are properly enforced - both food safety and surveillance - if BSE risks are to be effectively managed." He promised that a report on the investigations would be made public.
Sue Davies, principal policy officer for the Consumers' Association, said: "We are very concerned that so many cattle have been allowed into the food chain without testing, even if testing is mainly for surveillance purposes.
"These are the same bodies that would be given responsibility for testing over-30-month cattle if the rule is lifted. It is now clear that removal of that rule would be premature."