Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 36. Demilitarized Zones
The UK LOAC Manual (2004) states:
5.39.1. To qualify as a demilitarized zone, the following conditions should usually be fulfilled:
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. any activity linked to the military effort must have ceased.”
5.39.2. Demilitarized zones may only be set up by agreement between the parties to the conflict. Their precise status will depend on the terms of the agreement but, generally speaking, these zones will not be used by any party for the conduct of military operations. Their purpose is to protect the areas themselves, or the people in them, from hostilities or as refuges for the civilian population.
5.39.4. The agreement for the establishment of a demilitarized zone:
a. must be express;
b. but may be concluded orally or in writing;
c. may be made through direct contact with the opposing party or through a Protecting Power or an impartial humanitarian organization;
d. may consist of “reciprocal and concordant declarations”;
e. should “define and describe, as precisely as possible, the limits of the demilitarized zone and, if necessary, lay down the methods of supervision”;
f. may specify conditions to be complied with or provide interpretation of terms, for example, on the meaning of “fixed military installations”, “acts of hostility”, “activity linked to the military effort” or “purposes related to the conduct of military operations”, or specify perimeter signs or specify who may be admitted to the demilitarized zone or as to what is to happen in the event of a breach of any of the conditions or if the fighting draws near to the zone.
5.39.5. The party controlling the demilitarized zone is responsible for marking it with agreed signs, especially on its perimeter and on highways.
5.39.6. “If the fighting draws near to a demilitarized zone, and if the Parties to the conflict have so agreed, none of them may use the zone for purposes related to the conduct of military operations or unilaterally revoke its status.”
5.39.7. A material breach by one party of its obligations under sub-paragraphs 5.39.1 or 5.39.6 will release the other party from its obligations under the agreement but the zone will continue to benefit from any other protection available under international law. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 5.39.1–5.39.7; see also § 10.39.
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 LOAC Manual (2004) states:
It is prohibited for the Parties to the conflict to extend their military operations to zones on which they have conferred by agreement the status of demilitarized zone, if such extension is contrary to the terms of this agreement. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.39.
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.
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).