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Convention (II) for the Amelioration of the Condition of Wounded, Sick and Shipwrecked Members of Armed Forces at Sea. Geneva, 12 August 1949.
Discontinuance of protection
The protection to which hospital ships and sick-bays are entitled shall not cease unless they are used to commit, outside their humanitarian duties, acts harmful to the enemy. Protection may, however, cease only after due warning has been given, naming in all appropriate cases a reasonable time limit, and after such warning has remained unheeded.
In particular, hospital ships may not possess or use a secret code for their wireless or other means of communication.