Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 10. Civilian Objects’ Loss of Protection from Attack
The UK LOAC Manual (2004) states:
Civil aircraft may legitimately be used for support missions such as transporting troops or supplies. When so used they lose their protection and become legitimate military objectives. Whether civil aircraft on the ground are legitimate objects of attack depends upon whether they are legitimate military objectives. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 12.6.2.
The manual further states:
Conditions of exemption for civil airliners
12.31. Civil airliners are exempt from attack only if they:
a. are innocently employed in their normal role; and
b. do not intentionally hamper the movements of combatants.
12.32. If aircraft exempt from attack breach any of the applicable conditions of their exemption as set forth in paragraphs 12.29 to 12.31, they may be attacked only if:
a. diversion for landing, visit and search, and possible capture, is not feasible;
b. no other method is available for exercising military control;
c. the circumstances of non-compliance are sufficiently grave that the aircraft has become, or may be reasonably assumed to be, a military objective; and
d. the collateral casualties or damage will not be disproportionate to the military advantage gained or anticipated. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 12.31–12.32.
The manual also provides:
The following activities may render enemy civil aircraft military objectives:
a. engaging in acts of war on behalf of the enemy, e.g., laying mines, minesweeping, laying or monitoring sensors, engaging in electronic warfare, intercepting or attacking other civil aircraft, or providing targeting information to enemy forces;
b. acting as an auxiliary aircraft to an enemy’s armed forces, e.g., transporting troops or military cargo, or refuelling military aircraft;
c. being incorporated into or assisting the enemy’s intelligence gathering system, e.g., engaging in reconnaissance, early warning, surveillance, or command, control and communications missions;
d. flying under the protection of accompanying enemy warships or military aircraft;
e. refusing an order to identify itself, divert from its track, or proceed for visit and search to a belligerent airfield that is safe for the type of aircraft involved and reasonably accessible, or operating fire control equipment that could reasonably be construed to be part of an aircraft weapon system, or on being intercepted clearly manoeuvring to attack the intercepting belligerent military aircraft;
f. being armed with air-to-air or air-to-surface weapons; or
g. otherwise making an effective contribution to military action. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 12.37.
Furthermore, the manual states:
The following activities may render neutral civil aircraft military objectives:
a. if they are believed on reasonable grounds to be carrying contraband, and, after prior warning or interception, they intentionally and clearly refuse to divert from their destination, or intentionally and clearly refuse to proceed for visit and search to a belligerent airfield that is safe for the type of aircraft involved and reasonably accessible;
b. if they engage in belligerent acts on behalf of the enemy;
c. if they act as auxiliaries to the enemy’s armed forces;
d. if they are incorporated into or assist the enemy’s intelligence system; or
e. if they otherwise make an effective contribution to the enemy’s military action, e.g., by carrying military materials, and, after prior warning or interception, they intentionally and clearly refuse to divert from their destination, or intentionally and clearly refuse to proceed for visit and search to a belligerent airfield that is safe for the type of aircraft involved and reasonably accessible. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 12.43.1.
With regard to internal armed conflict, the manual states:
15.16.1. There is no definition of civilian objects nor is the term used in the treaties dealing with internal armed conflicts, but the principles of military necessity and humanity require attacks to be limited to military objectives. Thus attacks on the following are prohibited unless they are being used for military purposes: civilian dwellings, shops, schools and other places of non-military business, places of recreation and worship, means of transportation, cultural property, hospitals and medical establishments and units.
15.16.2. UN General Assembly Resolution 2675, which was unanimously adopted and applies to all armed conflicts, can be regarded as evidence of state practice. Paragraph 5 of the resolution states: “dwellings and other installations that are used only by the civilian population should not be the object of military operations.” The principle of military necessity demands that civilian property may only be destroyed, or requisitioned for use, for necessary military purposes.
15.16.3. The old practices that tolerated during sieges the bombardment of civilian buildings (other than places of worship), hospitals, cultural property, indispensable objects and works containing dangerous forces, have not survived. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.16.1–3.
In 2005, in reply to a question concerning the protection of cultural property by British forces in Iraq, the UK Parliamentary Under-Secretary of State, Foreign and Commonwealth Office, stated:
The UK is a signatory to the 1977 Additional Protocol (AP 1) of the 1949 Geneva Conventions, which requires that civilian objects are to be protected from attack, unless that object is being used for military purposes; in which case it may lose its special protected status. UK forces in Iraq have strict instructions not to fire on cultural sites such as mosques and heritage sites, as well as important economic and social infrastructure properties, unless these sites are being used for militarily offensive purposes. 
United Kingdom, House of Lords, Statement by the Parliamentary Under-Secretary of State, Foreign and Commonwealth Office, Hansard, 25 October 2005, Vol. 674, Debates, col. WA179.
The UK LOAC Manual (2004) states: “In cases of doubt, objects are to be considered as civilian.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.24.3.
In its chapter on air operations, the manual states: “In case of doubt whether a vessel or aircraft exempt from attack is being used to make an effective contribution to military action, it shall be presumed not to be so used.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 12.33.