Council investigators have found traces of pork in beef burgers on sale at a Birmingham butchers.
The discovery comes as city environmental health officers are stepping up food checks following the horse meat scandal.
More than 200 million products have been withdrawn from sale in the last two months since the Food Safety Authority found burgers supplied to supermarkets contained horse.
Firms including Findus, Ikea, Tesco, Aldi, Burger King, Asda and Co-op have all been affected by the saga.
The Ministry of Justice also suspended a prison meat supplier after it was discovered Halal pies and pasties may have contained traces of pork DNA. And schools in Sandwell, Staffordshire and Walsall have stopped serving some beef meals to pupils.
Last month Birmingham City Council carried out tests on 50 beef products sold at shops and butchers. The test results have now revealed beef burgers sold at one butchers in Birmingham – which cannot be named for legal reasons – contained traces of pork.
Nick Lowe, the council’s foods operations manager, said: “Fortunately, it was not a halal butchers and we found less than one per cent of the product was pork DNA.
“We believe this was caused by contamination, rather than deliberate substitution, and probably occurred when mincing machinery had not been cleaned properly.”
He said the department had warned the company to carry out a thorough clean of its products.
Meanwhile, a further 112 products have been tested for horse meat and pork, with results yet to be revealed.
Mr Lowe added: “We are extending the range of products we are looking at to include all processed meat items, such as kebabs and ready meals.
“We are also moving our investigations across the production chain, looking at not just shops but also at wholesalers, manufacturers and cutting plants.”
He said a further round of tests will also be carried out at the end of this month.