Sandwich chain Subway has defended changing the contents of some products in line with demands by Muslims for meat to comply with sharia law.
Sandwiches at 185 Subway outlets in Britain and Ireland no longer come with ham or bacon as the pig is regarded as unclean in Islam.
The pork has been replaced by turkey in a move the company hopes will avoid offending Muslim customers. Turkey is halal, or clean, to Muslims.
Subway outlets adopting the new policy will display a window sticker reading: "All meats are halah."
But the practice of halal slaughter, which can only be carried out by Muslim men who chant "Bismillah Allahu-Akbar" (in praise of God) before the slaughter, is not without controversy. Critics say it is inhumane because sharia law demands the animal is conscious at the moment of slaughter.
Subway insisted that the animals that provided its meat were stunned before slaughter, as required by the EU. Some Muslim leaders feel that stunning an animal before it is slaughtered does not transgress the tenets of the Koran, the RSPCA said.
A Subway spokesman said: "All our suppliers comply with EU animal welfare legislation as a minimum and we require suppliers of halal products to adopt the stunning of animals prior to their slaughter. All halal meats are certified by the appropriate halal authorities.
"All halal Subway stores have numerous signs stating that they serve halal food. These are situated on the menu panels, nutritional information and in the front window of the store."
The US-based firm said it was responding to community requests. "The diverse multicultural population across the UK and Ireland means we have to balance the values of many religious communities with the overall aim of improving the health and welfare standards of animals," said a spokesperson.
An RSCPA spokesman said: "Scientific research has clearly shown that slaughter of an animal without stunning can cause unnecessary suffering, and the RSPCA is opposed to the slaughter of any animal without first making it insensible to pain and distress.
"The only legal exemptions to stunning during slaughter exist for kosher and halal methods of slaughter. However, it is important to differentiate between 'religious' and 'non-stun' slaughter as around 90% of all halal in the UK receives a stun before slaughter.
"Our concern has nothing to do with the expression of religious belief but with the practice of killing by throat cutting without pre-stunning.
"We believe that meat produced from animals stunned or not stunned before slaughter should be clearly labelled to allow consumer choice, and continue to press for changes in the law that would improve the welfare of all farm animals at the time of slaughter."