Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 85. The Use of Incendiary Weapons against Combatants
In 1976, during discussions in the Ad Hoc Committee on Conventional Weapons established by the CDDH, the representative of the United Kingdom stated:
18. His country had at present no requirement for napalm, but that it possessed other weapons capable of causing death by burning … His delegation could not subscribe to [a] prohibition [of these weapons].
19. The United Kingdom, which was seriously concerned about the suffering caused by flame weapons, was participating actively in negotiations designed to ascertain ways in which the international community might reduce such suffering… .
21. … Incendiary weapons could be both effective and discriminating …
22. The issue at stake was the right of States to use incendiary weapons when they felt their security threatened. It was not easy to deny them that right; but at the same time there was good reason to believe that the great majority of delegations at the current Conference would be happy to see some limitation on the use of such weapons … The Netherlands proposal [submitted as an annex to a working paper on incendiary weapons, see supra] provided an excellent basis for negotiation, and it was greatly to be hoped that the Committee would reach agreement along these lines. 
United Kingdom, Statement at the CDDH, Official Records, Vol. XVI, CDDH/IV/SR.28, 20 May 1976, p. 287, §§ 18–22.
In 2008, in response to a question in the House of Commons, the UK Secretary of State for Defence wrote:
In Afghanistan, white phosphorus munitions are routinely used to protect troops on operations by producing a smoke screen to provide cover. Records show white phosphorus munitions were last used for the same purpose in Iraq in 2005.
In accordance with the UN third convention on conventional weapons [i.e. Protocol III to the Convention on Certain Conventional Weapons], UK training in the use of white phosphorus emphasises that it should be used solely for its intended purpose. 
United Kingdom, House of Commons, Written Statement by the Secretary of State for Defence, Hansard, 23 February 2009, Vol. 488, Written Statements, col. 18W.
The UK Military Manual (1958) states: “The use of flame throwers when directed against military targets is lawful. However, their use against personnel is contrary to the law of war in so far as it is calculated to cause unnecessary suffering.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 110, footnote 1.
The UK LOAC Manual (2004) states:
6.12.1. Although [incendiary weapons] can cause severe injury to personnel, their use is lawful provided the military necessity for their use outweighs the injury and suffering which their use may cause.
6.12.6. Use of weapons such as napalm and flame-throwers against combatant personnel is not dealt with specifically in the Conventional Weapons Convention or any other treaty. Such uses are governed by the unnecessary suffering principle so that they should not be used directly against personnel but against armoured vehicles, bunkers and built-up emplacements, even though personnel inside may be burnt. The same applies to white phosphorous, which is designed to set fire to targets such as fuel and ammunition dumps or for use to create smoke, and which should not be used directly against personnel. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.12.1 and 6.12.6.
In 2008, in response to a question in the House of Commons, the UK Secretary of State for Defence wrote regarding operations in Afghanistan: “In accordance with the UN third convention on conventional weapons [i.e. Protocol III to the Convention on Certain Conventional Weapons], UK training in the use of white phosphorus emphasises that it should … not [be used] as an anti-personnel weapon.” 
United Kingdom, House of Commons, Written Statement by the Secretary of State for Defence, Hansard, 23 February 2009, Vol. 488, Written Statements, col. 18W.
In 1973, in response to Resolution 2932 A (XXVII) in which the UN General Assembly asked States to comment on the report of the UN Secretary-General on napalm and other incendiary weapons and all aspects of their possible use, the United Kingdom emphasized that incendiary weapons must not be used to create unnecessary suffering and recommended further study of this issue. 
United Kingdom, Reply sent to the UN Secretary-General, reprinted in Report of the Secretary-General on napalm and other incendiary weapons and all aspects of their possible use, UN Doc. A/9207/rev.1 add.1, 11 October 1973.