Council bosses have confirmed a halal lamb burger made by a company supplying Leicester schools contained up to 50 per cent pork.
The burgers were removed from 19 city school menus after the tests revealed the presence of pork in a sample.
The product was made by Paragon Quality Foods, of Doncaster, which said it had never knowingly bought or handled pork.
The discovery was made on April 18 and the burgers were withdrawn immediately. Details became public on Thursday.
A DNA test found the burger contained between 10 and 50 per cent pork.
The burgers had been on Leicester school menus since January and are the only product supplied to the council by the Yorkshire firm.
Trevor Pringle, the council director responsible for school meals said: “We have only received the DNA test result for one burger to date and we shared this with the Federation of Muslim Organisations.
“To get a better understanding, we have asked for burgers from different batches to be analysed.
“We expect those to be back some time next week.
“We are clear, however, that any level of pork content in this product is unacceptable.”
All other halal products used in council kitchens are supplied by The Punjab Kitchen Ltd, of Tyneside. Tests have shown it is Halal compliant.
The council sent 6,000 letters to parents and head teachers at the affected schools.
Metin Pekin, of Paragon Quality Foods, said the company was a “pork-free site and had never knowingly bought or handled pork”.
He said: “We have carried out full traceability of the product in question and have provided this information to the relevant enforcement authorities.”