In response to queries from the NSS, the House of Commons said: "Both halal and kosher meals sold by the House contain meat from animals not pre-stunned before slaughter."
Parliament rolled out its new halal and kosher menu last month, after Labour MPs Charlotte Nichols and Zarah Sultana coordinated a cross party letter to MPs urging them to write to the parliamentary authorities asking them to provide kosher and halal food.
Most animals killed by the halal method in the UK are stunned beforehand.
However, some sections of the Muslim community do not accept an animal as 'halal' if it has been stunned.
Parliament's decision to supply halal meat from the less common non-stun slaughter method has been criticised by the NSS.
All kosher meat certified in the UK is from animals that were slaughtered without stunning.
UK law requires animals to be stunned before they are slaughtered to avoid unnecessary suffering, distress or pain. However, an exemption allows sheep, goats, cattle and poultry to be slaughtered without pre-stunning for religious purposes.
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