Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 72. Poison and Poisoned Weapons
The UK Military Manual (1958) states:
Poison and poisoned weapons … are forbidden.
Water in wells, pumps, pipes, reservoirs, lakes, rivers and the like, from which the enemy may draw drinking water, must not be poisoned or contaminated. The poisoning or contamination of water is not made lawful by posting up a notice informing the enemy that the water has been thus polluted. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 111 and 112.
The manual also provides:
In addition to the “grave breaches” of the 1949 Geneva Conventions, … the following are examples of punishable violations of the laws of war, or war crimes: … using … poisoned … arms or ammunition; … poisoning of wells, streams, and other sources of water supply; … using … poisonous … gases. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(g), (i) and (r).
The UK LOAC Pamphlet (1981) states: “The following are prohibited in international armed conflict: … c. poison and poisoned weapons.” 
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 5, p. 20, § 1(c); see also Section 4, p. 12, § 2(e).
(emphasis in original)
The UK LOAC Manual (2004) states:
6.19. It is prohibited “to employ poison or poisoned weapons”.
6.19.1. The prohibition applies to any use of poison, including the poisoning or contamination of water supplies. Such poisoning or contamination would not be made lawful by the posting of a notice informing the enemy of the fact. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.19–6.19.1.
In its chapter on internal armed conflict, the manual prohibits the use of poison or poisoned weapons. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.28.
In its chapter on enforcement of the law of armed conflict, the manual states:
The Hague Regulations 1907 are now recognized as part of customary law. Those regulations provide that the following acts are “especially forbidden”:
a. to employ poison or poisoned weapons. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.27.
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(xvii) 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).
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom referred to the “long established prohibition on the use of poison and poisoned weapons”, but it also stated that the prohibition was “intended to apply to weapons whose primary effect was poisonous and not to those where poison was a secondary or incidental effect”. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, §§ 3.59 and 3.60.