Pork DNA has been found in halal chicken sausages which have been served in at least one London primary school, Westminster council has confirmed.
Other tests revealed that lean mince supplied to the council also contained traces of lamb and pork DNA.
The contaminated chicken was found at St Mary's Bryanston Square, a Church of England school in west London, Westminster City Council confirmed.
Both items have been removed from all Westminster school menus and supplies from the contractor have been put on hold.
Westminster ordered tests to be carried out in the wake of the horsemeat scandal.
All tests for horsemeat in the food came back negative but a preliminary report from the Public Analyst Scientific Services revealed pig DNA in the halal chicken sausages range.
The sausages were also a menu choice in 14 other primary schools, two nurseries, one special school and one pupil referral unit.
Andrew Christie, Westminster City Council's director of children's services, said: "We are very concerned by the discovery that a contractor has fallen short of the high standards we demand.
"We also understand and regret the upset that may have been caused to parents and children alike. We are contacting schools, parents and faith group leaders and are offering to meet them to discuss their concerns.
"Westminster City Council took prompt action when this issue came to light and immediately ensured this product stopped being served in the schools in question. We have asked our contractor Chartwells to no longer use the meat supplier involved.
"The discovery of pork in these sausages came about because of extra tests Westminster City Council decided to carry out. The results were all the more shocking given the assurances about the food we receive from our suppliers. I can assure parents we will keep testing our school meals."
The East London Mosque & London Muslim Centre said: "We are deeply concerned about recent developments with the discovery of pork DNA in halal school meals served to Muslim primary school children.
"It is strictly prohibited for Muslims to eat pork or meat that has not been slaughtered in accordance with their faith and thus the discovery of pork meat in Halal sausages will be hard to 'stomach' for the parents of the children that have been exposed to this contaminated meat.
"There will be serious issues of concern and distrust between Muslim parents and the schools now, and we hope a resolution can be sought quickly and effectively."
In February, the Food Standards Agency also found traces of pork in meat supplied to prisons.