Pork DNA has been found in halal chicken sausages served by caterer Chartwells in a Westminster primary school, the local authority has said.
The discovery was made after Westminster Council decided to carry out its own tests on food served in its schools in the wake of the horse meat scandal.
It took seven samples of meat products in total from three schools in the last week of February and tested them for the presence of DNA from beef, lamb, pork, chicken, turkey, goat and horse. All proved negative for horse DNA.
The contaminated sausages were served at St Mary's Bryanston Square, a Church of England school, while tests on a sample from Burdett Coutts School revealed the presence of lamb and pork DNA in lean minced beef.
All items have been removed from all school meals and Chartwells, the education arm of Compass Group, was asked not to use its meat supplier.
In a statement, a spokesperson for Chartewlls said: "This product was supplied to Chartwells by butchers Nigel Fredericks from whom we sought and received written assurances that the product was correctly certified as Halal chicken.
"Nigel Fredericks has taken full responsibility for this breach of our supply chain and we are conducting our own investigation into how this happened. We immediately took the precautionary step of withdrawing the product from all sites and have issued unreserved apologies.
"We are deeply concerned by the finding and that, despite the written assurances we received, we have had this breach of our supply chain.
"The traceability and quality of our food is our top priority. All of our nominated suppliers have to adhere to strict food quality and safety standards and they regularly undergo independent audits to ensure these standards are upheld."
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.
"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."
A Defra spokesman said: "It is totally unacceptable if food labelled as halal has been found to contain pork.
"Today Defra, the FSA and the Department for Communities and Local Government met with organisations involved in halal and kosher food to discuss labelling and certification.
"Food manufacturers and suppliers have a responsibility to make sure that the food they serve is what they say it is. The FSA is working with Westminster City Council to establish what happened."
The Food Standards Agency was alerted by the council on Monday.