A/HRC/27/48 universal periodic review process of the Human Rights Council and other State-to-State peer review and dialogue-based mechanisms are providing increasing assistance to States in their compliance with international law obligations. 1. Preventive detention 75. In a number of countries, the Working Group has observed the use of preventive detention. Convicted persons who have served their sentence may continue to be deprived of their liberty on the basis that their release would pose a danger to society (see, for example, A/HRC/7/4/Add.2 and A/HRC/19/57/Add.3). When a criminal sentence includes a punitive period followed by a preventive period, the International Covenant on Civil and Political Rights and customary international law, as confirmed by the reports and constant jurisprudence of the Working Group, require that once the punitive term of imprisonment has been served, the preventive detention must be proportionate and justified by compelling reasons. It must be subject to regular periodic review by a court or by an independent body subject to judicial review that must be able to determine the continued justification for the detention. Strong procedural guarantees must be provided for the evaluation of future danger, and the proportionality review is no less strict at that stage and demands stronger reasoning as time passes. Post-conviction preventive detention must be a measure of last resort. The conditions in preventive detention must be distinct from the conditions for convicted prisoners serving a punitive sentence and be aimed at rehabilitation and reintegration into society. If a prisoner has served the sentence imposed at the time of conviction, articles 9 and 15 of the Covenant and customary international law forbid a retroactive increase in sentence. Detention equivalent to penal imprisonment cannot be imposed as civil preventive detention or under any other label. 2. Detention of asylum seekers and irregular migrants 76. Some States have resorted to administrative detention of asylum seekers and irregular migrants (see, for example, A/HRC/13/30/Add.2 and A/HRC/10/21/Add.5). Law and policy vary from State to State, and asylum seekers and irregular migrants are at risk of arbitrary detention. They may be detained for several months or years, or even indefinitely, particularly in countries which have a policy of mandatory detention or do not prescribe a maximum period of detention. The imprisonment of a migrant or an asylum seeker for a prolonged period of time, in conditions that are sometimes found to be even worse than in the regular prisons, constitutes a punishment on a person who has not committed any crime (A/HRC/27/48/Add.2, para. 75). 77. Other international human rights mechanisms have similarly raised concerns about the excessive length of detention of migrants, the harsh conditions of their detention and the lack of procedural safeguards to ensure that detention is an appropriate and proportionate measure (A/68/261, para. 46). The Working Group reiterates that the detention of asylum seekers and irregular migrants should be a last resort and permissible only for the shortest period of time. Any necessary detention should take place in appropriate, sanitary, nonpunitive facilities, and should not take place in prisons. The inability of the authorities to carry out the expulsion of an individual can never justify indefinite detention. C. Protective custody 78. The present section addresses the practice of keeping girls and women in detention for the purpose of protecting them from risks of serious violence. The Working Group has previously addressed in its annual report protective custody of women and girls who may be detained for life. That form of deprivation of liberty is highly gendered in its reach, remit and application. In some countries, women and girls are placed in custody due to the risk of 22