The company which supplied halal food found to contain traces of pork DNA to prisons 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 County Tyrone 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 company spokeswoman said it was "shocked" to find pork DNA traces.
Under Islamic law, Muslims are required to eat halal food - and eating pork is strictly forbidden.
The 3663 spokeswoman added the affected products had been withdrawn from supply and were only distributed to prisons.
"The products in question are from one particular food manufacturer, McColgan's Quality Foods Limited, and have not been distributed to any other customer. All halal products from this manufacturer have been withdrawn"
3663 said initial tests were 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 recognised a potential connection between a supplier of halal savoury pie products for the Ministry of Justice and one of their halal beef suppliers mentioned with the FSAI report," the company said.
3663 said horsemeat was not found but "disappointingly, we received evidence that within the products tested there were traces of porcine protein.
McGolgan's, based in County Tyrone since the mid-1970s, employs about 100 people at its site on the Dublin Road, Strabane.
A spokesman for McColgan's said: "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.
Steve Wearne, from the Food Standards Agency, said it had called an urgent meeting "of a range of suppliers" on Monday where it would "stress again the responsibility of all food businesses to ensure the food that they sell contains what it says on the label".
Halal meat is defined as meat slaughtered by hand and blessed by the person doing the killing, however some Muslims believe a mechanised form is also now acceptable.
The editor of the Muslim News, Ahmed Versi, said: "This is very serious because no Muslim would ever eat pork meat - anything to do with pork - and it must be very distressing for those in prison who have been given this meat to realise they may have been eating food which was contaminated with pig."