The prohibition of forced labour is an established norm of customary international law and is reflected in a series of international instruments, including the Universal Declaration of Human Rights and International Covenant on Civil and Political Rights. Forced labour is most clearly defined in the Forced Labour Convention as “all work or service which is exacted from any person under the threat of a penalty and for which the person has not offered himself or herself voluntarily.” An exception may be exercised for prison labour, but only under certain conditions, namely: “any work or service exacted from any person as a consequence of a conviction in a court of law, provided that the said work or service is carried out under the supervision and control of a public authority and that the said person is not hired to or placed at the disposal of private individuals, companies or associations.” Compulsory labour performed by detainees is excluded from the scope of the Forced Labour Convention only if a certain number of conditions are met. Most significantly for the case of forced labour in penal facilities, work can only be exacted from a detainee “as a consequence of a conviction in a court of law.” Detainees who have been subjected to arbitrary deprivation of liberty, who are awaiting trial, who have not been sentenced, or who have been sentenced without trial cannot be forced to perform labour.