Pork DNA has been found in Halal chicken sausages that have been served in at least one primary school, Westminster Council said on Thursday.
Other tests revealed that lean minced beef supplied to the council showed evidence of lamb and pork DNA.
The council said it immediately ensured both items were removed from all school menus and had instructed its contractor to temporarily cease supplying meat until the issue was resolved.
In the meantime, the council said it had asked its contractor to no longer use its Halal meat supplier.
The problem came to light after the authority decided to carry out its own proactive tests on food contamination in the wake of the horse meat scandal.
The council said officers took seven samples in total from three schools in the week beginning February 25, 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 beef, lamb, pork, chicken, turkey, goat and horse, all 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 result was found in one sample taken at a Westminster primary school.
A further round of tests today 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 its own investigation to establish how the contamination arose.
Westminster City Council's tri-borough director of children's services Andrew Christie 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 alerted the Food Standards Agency on March 11 in accordance with the usual protocols for incidents of this kind.