Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 84. The Protection of Civilians and Civilian Objects from the Effects of Incendiary Weapons
The UK LOAC Manual (2004) states:
6.12. It is prohibited “in all circumstances to make any military objective located within a concentration of civilians the object of attack by air-delivered incendiary weapons”. Subject to that, the use of incendiary weapons, including incendiary bombs, napalm and flame-throwers, against military objectives is not prohibited under customary or treaty law.
6.12.2. An incendiary weapon is “any weapon or munition which is primarily designed to set fire to objects or to cause burn injury to persons through the action of flame, heat, or a combination thereof, produced by a chemical reaction of a substance delivered on the target”.
6.12.3. Such weapons can take the form of, for example, “flamethrowers, fougasses, shells, rockets, grenades, mines, bombs and other containers of incendiary substances”.
6.12.4. The following are not considered incendiary weapons: “munitions which may have incidental incendiary effects, such as illuminants, tracers, smoke or signalling systems;” nor do they include “munitions designed to combine penetration, blast or fragmentation effects with an additional incendiary effect, such as armour-piercing projectiles, fragmentation shells, explosive bombs and similar combined-effects munitions in which the incendiary effect is not specifically designed to cause burn injury to persons, but to be used against military objectives, such as armoured vehicles, aircraft and installations or facilities”.
6.12.5. A “concentration of civilians” is widely defined to include the inhabited parts of cities or inhabited towns or villages but also temporary concentrations as in camps or columns of refugees or evacuees, or groups of nomads. An attack on a military objective in a concentration of civilians with incendiary weapons other than air-delivered ones may only take place if the military objective is clearly separated from the concentration and all feasible precautions are taken to limit the incendiary effect to the military objective and to avoiding or minimizing incidental loss, injury and damage to civilians. Forests and “other kinds of plant cover” may not be attacked with incendiary weapons unless they are “used to cover, conceal or camouflage combatants or other military objectives, or are themselves military objectives”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.12 and 6.12.2–6.12.5.
In its chapter on air operations, the manual further states: “It is in all circumstances prohibited ‘to make any military objective located within a concentration of civilians the object of attack by air-delivered incendiary weapons.’” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 12.53.
Upon ratification of the 1980 Convention on Certain Conventional Weapons, the United Kingdom stated:
The United Kingdom accepts the provisions of article 2(2) and (3) on the understanding that the terms of those paragraphs of that article do not imply that the air-delivery of incendiary weapons, or of any other weapons, projectiles or munitions, is less accurate or less capable of being carried out discriminately than all or any other means of delivery. 
United Kingdom, Declarations made upon ratification of the Convention on Certain Conventional Weapons, 13 February 1995, § d.
At the Preparatory Conference for the Convention on Certain Conventional Weapons in 1979, the United Kingdom stated:
The exchange of views in the past week had proved disappointing because some delegations had adopted extreme positions. These positions were doubtless dictated by humanitarian considerations which could in no sense be criticised. However, if agreement was to be reached on anything specific that would be an advance over the present state of the law, the objective would plainly have to be a more limited one. The text of the proposal submitted by Australia and the Netherlands met precisely that need. 
United Kingdom, Statement at the Preparatory Conference for the Convention on Certain Conventional Weapons, UN Doc. A/CONF.95/PREP.CONF/II/ SR.23, 6 April 1979, p. 2.
In 2009, in response to a question in the House of Common, the UK Secretary of State for Foreign and Commonwealth Affairs wrote:
We are very concerned about reports of white phosphorus ammunition being used by the Israeli defence force in Gaza. We have made this clear directly to both the Foreign Ministry and Defence Ministry in Tel Aviv.
I have also made clear to the House [of Commons] that Gaza is an exceptionally densely populated area where white phosphorus used as an air burst is liable to cause particularly horrific injuries to non-combatants. We consider such use in these circumstances unacceptable. 
United Kingdom, House of Commons, Written Statement by the Secretary of State, Foreign and Commonwealth Office, Hansard, 26 February 2009, Vol. 488, Written Statements, col. 951W.
In 2010, in its objection to the reservation by the United States of America to the 1980 Protocol III to the Convention on Certain Conventional Weapons, the United Kingdom stated:
[T]his reservation [in which the United States of America reserves the right to use incendiary weapons against military objectives located in concentrations of civilians where it is judged that such use would cause fewer casualties and/or less collateral damage than alternative weapons, but in so doing will take all feasible precautions with a view to limiting the incendiary effects to the military objective and to avoiding, and in any event to minimizing, incidental loss of civilian life, injury to civilians and damage to civilian objects] appears to be contrary to the object and purpose of the [1980] Protocol [III to the Convention on Certain Conventional Weapons] insofar as the object and purpose of the Protocol is to prohibit/restrict the use of incendiary weapons per se. On this reading, the United Kingdom objects to the reservation as contrary to the object and purpose of the Protocol.
The United States has, however, publicly represented that the reservation is necessary because incendiary weapons are the only weapons that can effectively destroy certain counter-proliferation targets, such as biological weapons facilities, which require high heat to eliminate the biotoxins. The United States has also publicly represented that the reservation is not incompatible with the object and purpose of the Protocol, which is to protect civilians from the collateral damage associated with the use of incendiary weapons. The United States has additionally stated publicly that the reservation is consistent with a key underlying principle of international humanitarian law, which is to reduce risk to the civilian population and civilian objects from harms flowing from armed conflict.
On the basis that (a) the United States reservation is correctly interpreted as a narrow reservation focused on the use of incendiary weapons against biological weapons, or similar counter-proliferation, facilities that require high heat to eliminate the biotoxins, in the interests of preventing potentially disastrous consequences for the civilian population, (b) the United States reservation is not otherwise intended to detract from the obligation to take all feasible precautions in the choice of means and methods of attack with a view to avoiding, and in any event to minimising incidental loss of civilian life, injury to civilians and damage to civilian objects, and (c) the object and purpose of the Protocol can properly be said to be to protect civilians from the collateral damage associated with the use of incendiary weapons, the United Kingdom would not object to the reservation as contrary to the object and purpose of the Protocol. 
United Kingdom, Objection to the Reservation by the United States of America to the 1980 Protocol III to the Convention on Certain Conventional Weapons, 4 February 2010.
In 2010, in a written answer to a question in the House of Lords about the status of weapons containing white phosphorus under international law and their use in densely populated civilian areas, a UK Minister of State, Foreign and Commonwealth Office, stated:
White phosphorous … [is] not prohibited under international law when used appropriately. However, Protocol III of the UN Convention on Certain Conventional Weapons prohibits or restricts the use of incendiary weapons against civilian populations and the use of air-delivered incendiary weapons against military objectives located in areas where there is a concentration of civilians, for those countries (including the UK) who are signatories to the protocol. 
United Kingdom, House of Lords, Written Answer by a Minister of State, Foreign and Commonwealth Office, Hansard, 30 March 2010, Vol. 718, Written Answers, col. WA370.