The Ministry of Justice has suspended a firm supplying pies and pasties to prisons after tests found that it may have provided products described as Halal - but with traces of pork DNA.
The Food Standards Agency (FSA) said the "local authority" involved was investigating, while the MoJ said the products have been withdrawn.
Justice minister Jeremy Wright said the incident was "absolutely unacceptable".
Under Islamic law, Muslims are strictly forbidden to eat pork.
In a statement, Mr Wright said the Prison Service was investigating the incident "as a matter of urgency".
He added: "This is an absolutely unacceptable situation and one which we regret greatly.
"Clearly this must be distressing for those affected and they can be reassured we are doing everything we can to resolve the situation."
An MoJ spokesman said that all the affected prisons have been informed.
"We are taking immediate steps to suspend the contract with the relevant subcontractor," he added.
A spokeswoman for the FSA said it was "informed" that a number of meat pies and pasties supplied to UK prisons - labelled and served as Halal - could have contained traces of pork DNA.
She said: "The local authority is investigating how this contamination came about and whether these products have been distributed further across the UK."
She added that the supplier and the local authority were based in the same area, but declined to say where.
Under Islamic law, the eating of pork is strictly prohibited for Muslims, as is the consumption of meat which has not been slaughtered in the correct way.
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.
Food and farming minister David Heath said people "have a right to expect that the food they are eating is correctly described".
He insisted: "I have made it clear that I want an urgent meeting with major retailers and suppliers first thing next week to get to the bottom of this completely unacceptable situation."