Traces of pork have been found in a sample of halal chicken sausages served to pupils in central London schools and nurseries, the local council revealed on Thursday.
Pork was present in one sample which may have been supplied by contractor Chartwells to 19 schools and nurseries, Westminster City Council said in a statement.
A further sample of non-halal lean mince beef supplied to 17 schools and nurseries also found pork and lamb.
The council commissioned the test in response to the recent scandal which saw supermarkets across Europe pull millions of frozen ready meals off the shelves after tests revealed that meat labelled as beef contained large quantities of horsemeat.
The council said it was "very concerned by the discovery.
"We understand and regret the concern that may be caused to parents and children," it added.
"We placed our trust in Chartwells and their suppliers and it is unacceptable that this has happened."
The central London authority has withdrawn all meat products from all schools under the Chartwell's contract until further notice and launched an investigation to find out the cause of the contamination.
Rabbi Debbie Young-Somers from West London Synagogue, responded: "For some members of the Jewish community it might be very troubling that this has occurred.
"Traditionally Jewish teaching holds that mistakes such as this made unknowingly hold no bearing on the person who ate the products. However as with all consumers we hope that in the future everyone will be able to make true informed choices about what they wish to eat," she added.
The positive chicken sample came from Church of England school St. Mary's Bryanston Square.
The Halal sausages were a menu choice in 15 primary schools, two nurseries, one special school and one pupil referral unit. The lean minced beef was used in 18 schools.