Halal lamb burgers have been withdrawn from a city's schools after tests revealed a sample contained pork.
The decision follows DNA tests on a batch of frozen burgers manufactured by Doncaster-based Paragon Quality Foods Limited in January, Leicester Council said.
Assistant city mayor Vi Dempster said: "I am appalled by this situation. It is disgraceful that none of us can have confidence in the food we eat. We regret any distress this will cause families and staff, and would like to reassure them that robust action is being taken to address this serious matter."
Trevor Pringle, Leicester City Council's director of young people's services, said: "We understand that since March 1 this year, the product supplier, Paragon Quality Foods Limited, has only been releasing products to customers after obtaining negative DNA results for pork.
"However, as a precautionary measure we have removed all of the burgers we had in stock from this supplier whilst further investigation is carried out, and we will not be purchasing any further products from this supplier. We have made it clear to our suppliers that this is totally unacceptable, and we are taking urgent legal advice about the next steps."
All other Halal products used in the council's kitchens - including 24 city schools - are supplied by another company, the council said. They have been DNA-tested and found to be compliant, the authority added.
Suleman Nagdi, from the Federation of Muslim Organisations (FMO), said: "The community will be extremely shocked and distressed to learn of the contamination that has taken place."
The Food Standards Agency and Doncaster Metropolitan Council have launched their own investigations and further tests will be taken of the remaining stock, Leicester City Council said. Adrian Russell, the council's director of environmental and enforcement services, said: "The city council is pressing the relevant authorities to investigate this matter as rigorously as possible. Food fraud is a serious offence and those responsible should face the full force of the law."
The council said it has contacted the headteachers of the schools potentially affected to advise them of the action being taken. It has also written to parents of children at the schools concerned and meetings have also taken place with Muslim city councillors.
Paragon Quality Foods said it had "never knowingly bought or handled pork" and it was working with the relevant authorities.