The first case of horsemeat in school dinners in mainland UK has been found by Food Standards Agency tests.
The horsemeat DNA was discovered in cottage pies delivered to 47 schools in Lancashire.
It has now been removed from kitchens and officials say children would have consumed only a "minute" amount of horsemeat.
But many tests of products from some of the major school food suppliers have come back negative.
Analysts at one of six UK laboratories carrying out the tests discovered the horsemeat late on Thursday. Head teachers in the area have been alerted to the finding.
Lancashire county councillor Susie Charles, cabinet member for children and schools, said: "We share the concerns people have about what is clearly a major problem in food supplies across the UK and Europe.
"Because of those concerns we decided to seek extra assurance that our external suppliers were not providing any products containing horsemeat DNA, and one of the products has returned a positive result.
"Relatively few schools in Lancashire use this particular product but our priority is to provide absolute assurance that meals contain what the label says - having discovered this one doesn't, we have no hesitation in removing it from menus.
"This does not appear to be a food safety issue but I've no doubt parents will agree we need to take a very firm line with suppliers and it is a credit to our officers that we have been able to quickly identify the problem and take the product off the menus."
Food catering giant Compass, which supplies many schools and other institutions with food products, has found horsemeat in some of the burgers it sold as beef in Northern Ireland and Ireland. But it has stressed none of these products reached its customers in mainland UK.
The burgers were supplied to a small number of Compass clients, the firm said, including two colleges which it will not name. The meat was supplied to Compass by Rangeland Foods, it said, which had claimed its products were free from horse DNA.
A Compass spokesman said: "This is totally unacceptable. We have informed all of the affected sites of these developments, explained the actions we have taken and issued unreserved apologies.
"We are deeply concerned by this finding and that, despite the written assurances we and our supplier received, we have had this breach of our supply chain."
The news comes as meat samples are being tested across the UK and beef is being removed from some school menus as a precaution.
One of the largest suppliers of meat products to schools, Brakes, said equine DNA had been found in one of its lasagnes produced by another firm Creative Food, from meat supplied by Hampshire-based Pinnacle Foods. But these lasagnes had only been supplied to one customer, Whitbread, and the product is therefore unlikely to be in school canteens.
Eden Food Service, which supplies schools in the Bristol area, said: "We are extremely confident that no contamination has occurred in any of our beef products as a result of robust due diligence processes we have in place with our suppliers.
"However, as an added precaution we are removing a small number of beef products from sale until we have received the satisfactory guarantees from our suppliers."
It comes after Staffordshire County Council, which provides meals to 87% of its schools, decided to take beef products off its menus. It said there was no suggestion there had been a problem with adulteration.
Two Northern Ireland education boards, North Eastern and South Eastern, have also withdrawn beefburgers from school menus over the horsemeat issue.
A third education board that had also withdrawn burgers - the Southern board - later reinstated them after "confirmation from suppliers that all meat supplied is from a bovine source".
England's Local Authority Caterers Association said it was in touch with the major suppliers of meat to schools to seek reassurances that their products were free from horse DNA.
It added that school meals remained nutritious and healthy and a good alternative to packed lunches.