[Exakt]

They never were "officially" dropped, but in the British coastal areas and US coastal waters, in February 1940, attacks on unidentifiable ships (blacked out) were authorized, has long as they were done while submerged, to give the impression that the ship would have hit a mine. In the rest of the world, the official BdU standing orders, were to follow the Prize Regulations, but like I said, it is very difficult for a U-boat to fully adhere to these regulations. Unofficially, Germany was conducting an unrestricted submarine warfare.