United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 99. Deprivation of Liberty
Section B. Deprivation of liberty in accordance with legal procedures
The UK Military Manual (1958) states:
The decisions regarding such assigned residence or internment [for imperative reasons of security] can only be made in accordance with a regular procedure to be prescribed by the Occupant in accordance with the obligations of [the 1949 Geneva Convention IV].
In 2010, in its closing submissions to the public inquiry into the circumstances surrounding the death of Baha Mousa and the treatment of those detained with him by UK armed forces in Iraq in 2003, the UK Ministry of Defence stated:
12. The treaties setting out rules of IHL are supplemented by rules of customary international law (CIL), i.e. rules which are recognized as binding by States, even though they do not appear in treaty texts. … [I]n relation to the rules described below the Government accepts that they reflect CIL. It is suggested that the rules which are of most relevance to this inquiry are:
12.12. … Arbitrary deprivation of liberty is prohibited.
12.13. The ICRC commentary to this rule distinguishes between international armed conflicts and non-international armed conflicts. In relation to the former, it refers exclusively to the internment regime set out in GC4 [the 1949 Geneva Convention IV] in relation both to the substantive right to intern and procedural protections during internment. The ICRC’s commentary on the  additional protocols recognises that the procedural standards which apply during preventive detention may have to differ during wartime from standards which apply to peacetime detention.
[footnote in original omitted; emphasis in original]