"Monarch" was paid off on 13 June 1923 and she was converted into a target ship at HM Dockyard, Portsmouth in October 1923. A replica of the underwater side protection of the s added to test the blast effects of large bombs detonating alongside the hull. A device consisting of of TNT (representing the explosives in a aerial bomb) was exploded at a depth of and from her side. Although damaged, the structure remained watertight after the test. The following year, live bombs of various sizes were dropped on the ship by Airco DH.9A bombers with little effect. Bombs were then positioned and detonated internally to evaluate their effects. On 20 January 1925, "Monarch" was towed out from Plymouth by dockyard tugs into Hurd's Deep in the English Channel, approximately south of the Isles of Scilly. On 21 January she was attacked by a wave of Royal Air Force bombers, which scored several hits; this was followed by four light cruisers firing six-inch (152-mm) shells, and the destroyer , using her 4-inch guns. For the finale, the fast battleships and and the five s opened fire with their 15-inch (381-mm) guns. The number of hits on "Monarch" is unknown, but after nine hours of shelling she finally sank at 22:00. after a final hit by .