British and French warships ordered to open at once fire on unsufficiently escorted submarines according to the Kriegsnachrichten of the Chef des Admiralstabes der Marine dated Berlin 16 August 1917

An item referred to a message of the German navy attache at Madrid, Spain dated 13 August reporting that a Spanish captain was informed by a French navy officer that all British and French warships were ordered to fire immediately at each submarine which was not escorted by at least two escort vessels.

Source

Bundesarchiv RM-40-622