HMS STERLET (Lt Cdr G H S Haward) sailed on 8 April 1940 to a patrol area off the Swedish coast. On 12 April 1940 she reported having unsuccessfully attacked a convoy west of the Skaw. She was redirected to a patrol area off Soet Fjord and is believed to have arrived at about midday on 13 April.
The German gunnery ship Brummer was attacked by a submarine that, it is believed, must have been STERLET on 15 April. One torpedo struck Brummer, and her escorts searched the area for a firing submarine. Although there is no definitive account from 1940 describing the loss of STERLET, it would appear she was most likely depth-charged by enemy antisubmarine vessels.
HMS STERLET was noted overdue on 27 April 1940 and subsequently reported as lost with All Hands. It is now thought that she was most likely sunk on 18 April 1940 in the Skagerrak south of Larvik in Norway in position 58 55’N 10 10’E after an attack by the German anti-submarine trawlers UJ-125, UJ-126 and UJ-128, although it is still also possible that she could have survived that attack only to be mined while returning to base.