Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 146. Reprisals against Protected Persons
Section E. Civilians in general
The UK LOAC Pamphlet (1981), in a part dealing with the protection of civilians, states: “It is forbidden: … to carry out reprisals against protected persons or property.” 
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(e).
The Pamphlet further states: “The Geneva Conventions and [the 1977 Additional Protocol I] prohibit reprisals against … enemy civilians in territory controlled by a belligerent.” 
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. 17, § 16.
However, the Pamphlet also states:
The United Kingdom reserves the right to take proportionate reprisals against an enemy’s civilian population or civilian objects where the enemy has attacked our own civilians or civilian objects in violation of [the 1977 Additional Protocol I]. 
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. 17, § 17.
The UK LOAC Manual (2004) states: “Additional Protocol I extends the categories of persons and objects against whom reprisals are prohibited to: a. civilians and the civilian population”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.19.1.
The manual also restates the interpretative declaration made by the UK upon ratification of the 1977 Additional Protocol I. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.19.1.
The manual explains:
This means that reprisals taken in accordance with the statement are permissible by and against the United Kingdom. However, commanders and commanders-in-chief are not to take reprisal action on their own initiative. Requests for authority to take reprisal action must be submitted to the Ministry of Defence and require clearance at Cabinet level. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.19.2.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated:
To be lawful, a belligerent reprisal must meet two conditions. First, it must not be directed against persons or objects against which the taking of reprisals is specifically prohibited … The Geneva Conventions of 1949 prohibit the taking of reprisals against persons or objects protected by the Conventions … The Conventions do not preclude the taking of reprisals against the enemy’s civilian population … Additional Protocol I prohibits the taking of reprisals against the civilian population (Article 51(6)) … The application of these provisions would have a greater effect on the retaliatory use of nuclear weapons. Again, however, these provisions are correctly regarded as innovative and thus as inapplicable to the use of nuclear weapons. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, pp. 58–59.
Upon ratification of the 1977 Additional Protocol I, the United Kingdom stated:
The obligations of Articles 51 and 55 are accepted on the basis that any adverse party against which the United Kingdom might be engaged will itself scrupulously observe those obligations. If an adverse party makes serious and deliberate attacks, in violation of Article 51 or Article 52 against the civilian population or civilians or against civilian objects, or, in violation of Articles 53, 54 and 55, on objects or items protected by those Articles, the United Kingdom will regard itself as entitled to take measures otherwise prohibited by the Articles in question to the extent that it considers such measures necessary for the sole purpose of compelling the adverse party to cease committing violations under those Articles, but only after formal warning to the adverse party requiring cessation of the violations has been disregarded and then only after a decision taken at the highest level of government. Any measures thus taken by the United Kingdom will not be disproportionate to the violations giving rise thereto and will not involve any action prohibited by the Geneva Conventions of 1949 nor will such measures be continued after the violations have ceased. The United Kingdom will notify the Protecting Powers of any such formal warning given to an adverse party, and if that warning has been disregarded, of any measures taken as a result. 
United Kingdom, Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § (m).