Frozen halal lamb burgers have been withdrawn from schools in Leicester after a batch tested positive for pork DNA.

Leicester City Council said a sample taken from a batch of frozen lamb burgers produced in January by Doncaster-based Paragon Quality Foods had tested positive for pork DNA. A spokeswoman was not immediately able to say how much pork DNA was found but said it was “more than a trace”. Further results were expected next week, she added.

Paragon has had a positive release system in place since 1 March, only releasing products once they have tested negative for pork, but the council said it had nevertheless decided to take the burgers off school menus as a precaution and would not be buying further products from Paragon while investigations were pending.

“We have made it clear to our suppliers that this is totally unacceptable, and we are taking urgent legal advice about the next steps,” said Trevor Pringle, the city council’s director of young people’s services.

The council has written to the parents of children at the schools affected by the withdrawal, which took place on 19 April, and said it was working with the Federation of Muslim Organisations (FMO) as part of its investigations.

Suleman Nagdi of the FMO said Muslims would be “extremely shocked and distressed” to hear pork DNA had been found in halal products. “The FMO is working closely with the local authority and calling on them to take legal action in respect of this contamination and would urge the local authority to instigate criminal proceedings against the company involved under the Food Safety Act,” he added.

The lamb burger was the only halal product supplied to Leicester schools by Paragon. Leicester City Council said all its other halal products used in schools had tested negative for pork DNA.

In February this year, pork DNA was found in supposedly halal pasties supplied to prisons by McColgan’s Quality Foods via 3663.

Paragon Quality Foods said it was a pork-free site and had never knowingly bought or handled pork. Previous samples of its products taken unannounced by Doncaster Borough Council had come back clear, it added. “We have carried out a full traceability of the product in question and have provided this information to the relevant enforcement authorities.”