Pork extracts have been found in a halal meat product during tests carried out by Birmingham City Council.
The council said the pork was found in packs of 20 Humza brand chicken burgers with a best before date of 10 May 2013.
There is no evidence to suggest Roshan Foods, who distributed the product, has caused the contamination deliberately, a spokesperson said.
The council said it was working closely other local authorities to trace "where and how this has occurred".
The pork extracts were found while checking for adulteration with horsemeat or other non-specified meat species, a spokesman added.
The council said investigations were "on-going" and the Birmingham-based company involved is "co-operating fully".
Their retail and catering customers have been notified and a product recall has started, the council said.
A spokesperson said further sampling on other products produced by the company has been carried out and the results should be known "by the end of next week".