Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 29. Medical Transports
Section C. Respect for and protection of hospital ships
The UK Military Manual (1958) states: “Convoys of vehicles or hospital trains on land, and specially provided vessels at sea, conveying wounded and sick civilians, the infirm, and maternity cases must be protected and respected in the same way as civilian hospitals.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 33.
The UK LOAC Manual (2004) states:
7.12. Medical transport means any means of transportation, whether military or civilian, permanent or temporary, assigned exclusively to the conveyance by land, water or air of the wounded, sick, shipwrecked, medical or religious personnel, medical equipment or medical supplies protected by the Geneva Conventions and Additional Protocol I and under the control of a competent authority of a party to the conflict. In Additional Protocol I, reference to “medical vehicles” expressly means “any medical transports by land”. Similarly, “medical ships and craft” means “any medical transports by water” and “medical aircraft” means “any medical transports by air”.
7.12.1. The assignment to medical purposes must be exclusive, although it may be permanent or temporary. The word “exclusive” is intended to restrict the definition of medical transport and its use so that the essential protection will not be eroded by abuses. “Permanent” means for an indeterminate period; “temporary” means limited periods but devoted exclusively to medical tasks during the whole of such periods.
Protection
7.20. The general rule is that medical transport is entitled to similar respect and protection as is given to medical units. However, there remain some practical difficulties especially in the case of medical ships and craft and medical aircraft. These categories are dealt with below.
Hospital ships
7.21. Ships that are built, converted or equipped specially and solely with a view to assisting the wounded, sick and shipwrecked and to treating them and transporting them are regarded as hospital ships. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 7.12–7.13.1 and 7.20–7.21.
In its chapter on maritime warfare, the manual provides that “hospital ships” and “small craft used for coastal rescue operations and other medical transports” are exempt from attack and from capture. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 13.33 and 13.100.
Lastly, in its chapter on enforcement of the law of armed conflict, the manual refers to “attacking a properly marked hospital ship or medical aircraft” as a war crime “traditionally recognized by the customary law of armed conflict”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.29.
The United Kingdom reacted to the sinking of the Tübingen during the Second World War by ordering an inquiry, in the course of which it was determined that, through a chain of errors on the part of the UK pilots and a misunderstanding in the wireless transmission, the order was actually given to attack the hospital ship. The UK Government expressed its regret at the sinking of the ship, stating:
In the circumstances described, they cannot refrain from remarking that had the Tübingen been properly illuminated at the time of sighting in accordance with international practice, the leader of the section would have had no difficulty in identifying her as a hospital ship and the incident would thus have been avoided. 
United Kingdom, as cited by Alfred M. de Zayas, The Wehrmacht War Crimes Bureau, 1939–1945, University of Nebraska Press, Lincoln, 1989, pp. 261–266.