Britain's Ministry of Justice suspended a supplier of halal meat to prisons on Friday after traces of pork were found inside meat pies and pasties, AFP
All prisons have been informed and the halal-labelled products were withdrawn immediately, an MoJ spokesman said.
"This is an absolutely unacceptable situation and one which we regret greatly," said justice minister Jeremy Wright.
"Clearly this must be distressing for those affected and they can be reassured we are doing everything we can to resolve the situation. The Prison Service is investigating this as a matter of urgency."
Islam's halal slaughter method requires the abattoir to kill the animal by slitting its throat. Islamic law forbids the consumption of pork.
The discovery comes after horsemeat was found in beefburgers stocked by major British supermarket chains, with one brand of burger coming in at 29 percent horse.
A spokeswoman for the Food Standards Agency said the latest incident involved traces of pork.
"The FSA has been informed that a number of meat pies and pasties supplied to UK prisons which were labelled and served as Halal contained traces of pork DNA," she said.
"The local authority (where the supplier is based) is investigating how this contamination came about and whether these products have been distributed further across the UK.
"People have a right to expect that the food they are eating is correctly described. We have called an urgent meeting of major retailers and suppliers on Monday to ensure that everyone is fully aware of their responsibilities.
"It is the responsibility of food businesses to ensure the food they sell contains what it says on the label. We are considering, with relevant local authorities, whether legal action is appropriate following the investigation."
The Prison Reform Trust campaign group welcomed the swift probe.
"This is not a matter of dietary preference but of Islamic law," said its director Juliet Lyon.
"There are clear hospital and prison rules that halal meat must be on the menu.
"This lapse will have offended and distressed high numbers of Muslim prisoners and their families so apologising, suspending the supplier and investigating the incident are the right steps for the MoJ to take."
Some 12.5 percent of the prison population of England and Wales are Muslims, according to MoJ figures from June 2011.