Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
The UK Military Manual (1958) defines an open or undefended town as:
A town which is so completely undefended from within or without that the enemy may enter and take possession of it without fighting or incurring casualties. It follows that no town located behind the immediate front line can be deemed open or undefended, since the attacker must fight his way to it. Any town behind the enemy front line is thus a defended town and is open to ground or other bombardment, subject to the conditions imposed on all bombardment, namely, that as far as possible, the latter must be limited to military objectives … A town in the front line with no means of defence, not defended from the outside and into which the enemy may enter and of which he may take possession at any time without fighting or incurring casualties, e.g., from crossing unmarked minefields, is undefended even if it contains munitions factories. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 290.
The manual goes on to say that, prima facie, a fortified place is considered as defended, unless there are visible signs of surrender. However, a locality need not be fortified to be deemed “defended”, and it may be held thus if a military force is occupying it or marching through it. It states that a town should be considered to be defended (and thus liable to bombardment) even if defended posts are detached and located at a distance from the city:
The town and defended posts form an indivisible whole, inasmuch as the town may contain workshops and provide supplies which are invaluable to the defence and may serve to shelter the troops holding the defence points when they are not on duty. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 289.
The UK LOAC Manual (2004) states:
5.38.1. The term “non-defended locality” has a special meaning. It is one where all the following conditions are met:
a. all combatants, as well as mobile weapons and mobile military equipment must have been evacuated;
b. no hostile use shall be made of fixed military installations or establishments;
c. no acts of hostility shall be committed by the authorities or by the population; and
d. no activities in support of military operations shall be undertaken.
5.38.2. This rule is an extension of the rule on undefended localities but depends on more formal arrangements. Where opposing ground forces are in contact, a commander might decide to withdraw from an inhabited area and allow the enemy to occupy it to avoid bloodshed among the civilian population or to preserve important historical or cultural sites. He can declare the place a non-defended locality.
5.38.3. The declaration of a non-defended locality should define as precisely as possible the limits of that locality and should be addressed to the adverse party who should acknowledge its receipt and then treat the locality as non-defended unless any of the conditions mentioned above is not fulfilled. In that event, it must inform the party making the declaration. It follows that a non-defended locality can be created without express agreement between the parties. It remains a non-defended locality until the party making the declaration withdraws it. Although no procedure for withdrawal is laid down, it should not take effect until notice of withdrawal has been given to the opposing party.
5.38.4. Even if all the conditions are not met, the parties may agree between themselves to treat an area as a non-defended locality. The agreement should be in writing and should specify the exact geographical limits of the locality, the date and time of the entry into force of the agreement and its duration, rules on marking the locality and agreed signs, persons authorized to enter the locality, methods of supervision (if any), whether and under what conditions the locality may be occupied by enemy troops.
5.38.5. If, in any case, an area is to be treated as non-defended, the party controlling it is responsible for marking it with agreed signs, especially on its perimeter and on highways. Some ingenuity will be required to devise signs visible from aircraft. Distinctive radio or electronic signals may be needed instead.
5.38.6. Even if it loses its non-defended status, the locality will benefit from other protection available under international law. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.38.1.–5.38.6.
With regard to internal armed conflict, the manual provides that (in addition to the prohibition on attacks against undefended localities) “[t]he other rules on protective zones applicable in international armed conflicts may be applied by analogy to internal armed conflicts”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.25.2.
The UK Military Manual (1958) states, with reference to Article 25 of the 1907 Hague Regulations, that the distinction between “defended” and “undefended” localities still exists and is not invalidated by the considerable destructive power of modern artillery and guided missiles. It clearly states the prohibition of any attack against undefended localities. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 290.
The manual further states: “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: … (c) firing on undefended localities”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(c).
The UK LOAC Pamphlet (1981) states that it is forbidden “to attack or bombard undefended towns, villages, dwellings or buildings”. 
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 4, p. 14, § 5(c).
The UK LOAC Manual (2004) states:
“The attack or bombardment, by whatever means, of towns, villages, dwellings, or buildings which are undefended is prohibited.” The reason for this rule is that there is no military need to attack a place that is not being defended. It can simply be occupied without resistance or bypassed.
“It is prohibited for the Parties to the conflict to attack, by any means whatsoever, non-defended localities.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.38; see also § 12.26 (air operations).
With regard to internal armed conflict, the manual states:
15.25. It is prohibited to attack or bombard, by whatever means, towns, villages, dwellings, or buildings that are undefended.
15.25.1. If a town, village, or building is undefended and can be occupied without resistance, there is no need to attack it and it is prohibited to do so. It is also prohibited to attack buildings unless they are military objectives. Undefended towns and villages that cannot be occupied because they are behind enemy lines or are in areas controlled by enemy forces may also not be attacked as such. Attacks against specific military objectives in those towns and villages are permitted, though precautions must be taken to minimize incidental loss or damage.
15.25.2. The other rules on protective zones applicable in international armed conflicts may be applied by analogy to internal armed conflicts. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.25–15.25.2.
In its chapter on enforcement of the law of armed conflict, the manual notes:
Additional Protocol I extends the definition of grave breaches to include the following:
b. any of the following acts, when committed wilfully, in violation of the relevant provisions of the protocol, and causing death or serious injury to body or health:
(4) making non-defended localities and demilitarized zones the object of attack. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.25.
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 … [the 1977 Additional Protocol I]”. 
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)(b)(v) 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).