Practice Relating to the Use of Prohibited Weapons
Germany’s Military Manual (1992) states: “Grave breaches of international humanitarian law are in particular: … use of prohibited weapons.” 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 1209.