Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 96. Hostage-Taking
The UK Military Manual (1958) forbids the taking hostage of the civilian population, whether in an occupied territory or not. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 42, 131, 554 and 650.
The manual specifies that “the taking of hostages” among persons protected by the 1949 Geneva Convention IV is a grave breach of that Convention. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 625(c).
The manual also mentions the killing of hostages in its list of war crimes. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(q).
The UK LOAC Pamphlet (1981) prohibits the taking of hostages, including in non-international armed conflicts. 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 9, p. 35, § 9 and Section 12, p. 42, § 2; see also Annex A, p. 48, § 20.
The UK LOAC Manual (2004) states in its chapter on the protection of civilians in the hands of a party to the conflict: “The following acts are prohibited “at any time and in any place whatsoever”: … c. the taking of hostages.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 9.4.
In its chapter on internal armed conflict, the manual restates the provisions of common Article 3 of the 1949 Geneva Conventions:
Under the terms of Common Article 3, the parties to a non-international armed conflict occurring in the territory of a party to the Conventions are obliged to apply “as a minimum”, the following provisions:
(1) Persons taking no active part in the hostilities, including members of armed forces who have laid down their arms and those placed hors de combat by sickness, wounds, detention, or any other cause, shall in all circumstances be treated humanely, without any adverse distinction founded on race, colour, religion or faith, sex, birth or wealth, or any other similar criteria.
To this end, the following acts are and shall remain prohibited at any time and in any place whatsoever with respect to the above-mentioned persons:
(b) taking of hostages. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.4.
In its chapter on enforcement of the law of armed conflict, the manual states:
In earlier times, hostages were often taken, given or exchanged to ensure observance of treaties, armistices and other agreements. The taking of hostages, whether civilian or military, is now prohibited. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.1.3.
The manual further notes:
Grave breaches under the Geneva Conventions consist of any of the following acts against persons or property protected under the provisions of the relevant convention:
h. taking hostages. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.24.
The UK Geneva Conventions Act (1957), as amended in 1995, punishes “any person, whatever his nationality, who, whether in or outside the United Kingdom, commits, or aids, abets or procures the commission by any other person of, a grave breach of any of the [1949 Geneva] conventions”. 
United Kingdom, Geneva Conventions Act, 1957, as amended in 1995, Section 1(1).
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(a)(viii) and (c)(iii) of the 1998 ICC Statute. 
United Kingdom, ICC Act, 2001, Sections 50(1) and 51(1) (England and Wales) and Section 58(1) (Northern Ireland).
The UK Government Strategy on the Protection of Civilians in Armed Conflict (2010) states: “IHL requires parties to a conflict to respect and protect civilians. … [C]ivilians must not be … subjected to acts of violence such as … hostage taking.” 
United Kingdom, Foreign and Commonwealth Office, Government Strategy on the Protection of Civilians in Armed Conflict, March 2010, p. 4.
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 regarding common Article 3 of the 1949 Geneva Conventions: “On its face this protection is restricted to armed conflicts not of an international character. However, it is understood to apply in all forms of armed conflict as part of customary international law to set out the irreducible minimum standard.” 
United Kingdom, Ministry of Defence, Closing Submissions to the Baha Mousa Public Inquiry on Modules 1–3, 25 June 2010, § 10.2, p. 10.