HMS E20 was built by Vickers, Barrow-in-Furness, and was commissioned on 30 August 1915. She was unusual in her class in having a 6-inch howitzer on the forward casing, intended for shore bombardment. Around 9 September, E20 sailed for the Mediterranean, arriving at HMS ADAMANT at Mudros on 24 September.

HMS E20 (Lt Cdr C Warren) was torpedoed in the Sea of Marmara by the German submarine UB14 on 5 November 1915. E20 had been due to rendezvous with the French submarine TURQUOISE, but the French boat had grounded and had been abandoned under fire. The enemy were able to capture information relating to the rendezvous and set a trap for E20.

UB14 lay in wait for the British boat and fired a torpedo at her from a range of 550 yards. Twenty-one men died in the resulting explosion but nine, including the Commanding Officer, were rescued from an oily patch on the surface. One of these survivors, AB A J Howard, subsequently died from chronic enteritis on 8 February 1917 while in captivity.