Muslims may be unwittingly eating pork, which is against their religion

The Food Standards Agency (FSA) said its surveys found that much of the chicken destined for the catering industry contained added protein.

This is not illegal, but processors have to declare it on the packaging, something the FSA found had not been done in a large number of cases.

The tests also found pork protein in half the catering chicken marked "Halal", denoting that the food can be eaten by Muslims, who are not supposed to eat pork under Islamic principles.

Added water

Concern over beef and pork additives follows an investigation screened by BBC One's Panorama programme, which followed research by the FSA which found high levels of added water in chicken.

There is currently no limit on the amount of water meat processors are allowed to add to compensate for liquid which is lost when the meat is cooked.

Beef and pork proteins are regularly added to chicken because this enables the product to hold very high levels of water, a common practice used to "pump up" the size of meat portions.

The FSA has recommended to the European Commission that it place a 15% cap on the amount of water allowed in processed poultry - mainly chicken and turkey.

If this proposal was made law, the need to add non-chicken proteins would be removed, although the FSA is also calling for a separate ban on adding non-chicken proteins to poultry products.

Customer rip-off

Sir John Krebs, chairman of the FSA, said: "Under European law it is not illegal to add huge amounts of water or beef and pork proteins to chicken as long as this appears on the label.

"It may be legal but it doesn't make it acceptable. The only reason to add proteins is to pump up the water to high levels and that's a recipe for ripping customers off.

"That is why we think that the amount of water that can be added to chicken should be limited and the use of non-chicken proteins banned."

The FSA's surveys found chicken pieces with added water accounting for up to 55% of the total weight.