The company accused of supplying halal food which contained pork has been named.
McColgan's Quality Foods Limited was the source of "the very small number of halal savoury beef pastry products," said food distributor 3663.
The Northern Ireland-based company said it was co-operating with The Food Standards Agency.
3663 carried out tests on five products after suspecting the halal products may have contained horsemeat.
A 3663 spokeswoman said it was "shocked" to find pork DNA traces.
Eating pork is strictly forbidden under Islamic law.
3663 added the affected products had been withdrawn from supply and were only distributed to prisons.
The initial tests were reportedly carried out on the products after the manufacturer's name appeared on a report by the Food Safety Authority of Ireland (FSAI) following an investigation which discovered horsemeat in some frozen burgers.
3663 said horsemeat was not found but "disappointingly, we received evidence that within the products tested there were traces of porcine protein.
A spokesman for McColgan's told the BBC: "McColgan's has already taken swift measures to identify, isolate and withdraw all of the products which are supplied to the Prison Service while an investigation to determine the circumstances surrounding this deeply regrettable and unforeseen incident takes place.
"McColgan's is keen to stress that at no point has pork of any kind been included in the recipes of any of the Halal-certified products it supplies."
The Food Standards Agency is now investigating whether the contaminated products were distributed more widely across the UK.BLOG COMMENTS POWERED BY DISQUS