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Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923.
Part II: Rules of Air Warfare#Chapter VI: Duties of belligerents towards neutral States and duties of neutrals towards belligerent States. - Art. 47.
Art. 47. A neutral State is bound to take the measures at its disposal to prevent aerial observations of the movements, operations and defense works of a belligerent from being made within its jurisdiction for the purpose of giving information to the other belligerent.
This rule also applies to a belligerent military aircraft on board a warship.