A local authority spokesman said: "The council has notified the Food Standards Agency, as it is required to do, and investigations are continuing. Our investigations are focusing on the use of frozen burger supplies during the past three months, the maximum length of time these would be held in storage." Samples of frozen mince came back negative for horse DNA.
Meanwhile, councils across the country were advised to take frozen beef burgers off the menu as a precautionary measure. The move was confirmed by procurement agency Scotland Excel, which deals with contracts on a national basis.
A spokesman said: "On the evening of February 21, we were advised by North Lanarkshire Council that traces of horse DNA were found in a frozen beef burger that had been supplied to one of the schools in their area.
"Scotland Excel is very concerned to learn of this incident despite the steps taken by our suppliers to provide assurances on their processed meat products. Investigations are taking place and we are continuing to work with our suppliers to ensure that the necessary verification is taking place across the supply chain.
"In the meantime, as a precautionary measure, councils have been advised to place a hold on the use of all frozen beef burger products currently stored within their catering premises."