Pig DNA has been found in halal chicken sausages that have been served in at least one primary school.
Westminster council said on Thursday that it had immediately ensured the item was removed from school menus, along with lean minced beef after other tests found evidence of pork and lamb in it.
The council said it had instructed its contractor to suspend meat supplies until the issue was resolved and to stop using its halal meat supplier.
The problem came to light after the authority carried out its own tests on food in response to the horsemeat scandal.
Westminster said officers took seven samples from three schools in the week beginning 25 February, adding that the tests "were carried out on our own initiative and not carried out as the result of any specific concerns about school food standards".
The testing, which involved analysis for the presence of DNA from cattle, lambs, pigs, chickens, turkeys, goats and horses, proved negative for horse DNA.
However, a preliminary report received on Friday suggested pig DNA could be present in the halal chicken sausages range. The evidence was found in one sample taken at St Mary's Bryanston Square primary. Further tests on Thursday confirmed the preliminary findings.
The halal sausages were a menu choice in 15 primary schools, two nurseries, one special school and one pupil referral unit, all of which took halal-only meat. The lean minced beef was used in 18 schools.
The authority said it had launched an investigation to establish how the contamination occurred.
Andrew Christie, Westminster city council's tri-borough 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 council said it had alerted the Food Standards Agency on 11 March in accordance with the usual protocols for incidents of this kind.
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."
In a separate development, supermarket chain Aldi said a random sample of its Oakhurst Frozen Meatloaf had been found to contain horsemeat.
An Aldi spokesman said: "Aldi has been contacted by the FSA to alert us that a random sample of our Oakhurst Frozen Meatloaf tested by a local authority has been found to contain horsemeat.
"We are surprised and deeply disappointed at this news. Our DNA testing, carried out last month in accordance with FSA guidance, also tested samples of this product and found it to be clear of horsemeat.
"The product is not part of our main range of everyday products and was stocked on a limited availability basis. However, we have immediately withdrawn any of the remaining stock from our stores.
"We will continue to test products and if we have any reason to believe the meat content is not correct, we will continue to act immediately in the interests of our customers.
"Customers can return the frozen meatloaf to their nearest Aldi for a full refund."