HMS Queen Mary was the last battlecruiser built by the Royal Navy before the First World War. The sole member of her class, Queen Mary shared many features with the Lion-class battlecruisers, including her eight 13.5-inch (343 mm) guns. She was completed in 1913 and participated in the Battle of Heligoland Bight as part of the Grand Fleet in 1914. Like most of the modern British battlecruisers, the ship never left the North Sea during the war. As part of the 1st Battlecruiser Squadron, Queen Mary attempted to intercept a German force that bombarded the North Sea coast of England in December 1914, but was unsuccessful. The ship was refitting in early 1915 and missed the Battle of Dogger Bank in January, but participated in the largest fleet action of the war, the Battle of Jutland in mid-1916. She was hit twice by the German battlecruiser Derfflinger during the early part of the battle and her magazines exploded shortly afterwards, sinking the ship.

Her wreck was discovered in 1991 and rests in pieces, some of which are upside down, on the floor of the North Sea. Queen Mary is designated as a protected place under the Protection of Military Remains Act 1986 as it is the grave of 1,266 officers and ratings.

The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against France. The modern Royal Navy traces its origins to the early 16th century; the oldest of the UK's armed services, it is consequently known as the Senior Service.

From the middle decades of the 17th century, and through the 18th century, the Royal Navy vied with the Dutch Navy and later with the French Navy for maritime supremacy. From the mid 18th century, it was the world's most powerful navy until the Second World War. The Royal Navy played a key part in establishing and defending the British Empire, and four Imperial fortress colonies and a string of imperial bases and coaling stations secured the Royal Navy's ability to assert naval superiority globally. Owing to this historical prominence, it is common, even among non-Britons, to refer to it as "the Royal Navy" without qualification. Following World War I, it was significantly reduced in size, although at the onset of World War II it was still the world's largest. During the Cold War, the Royal Navy transformed into a primarily anti-submarine force, hunting for Soviet submarines and mostly active in the GIUK gap. Following the collapse of the Soviet Union, its focus has returned to expeditionary operations around the world and it remains one of the world's foremost blue-water navies.The Royal Navy maintains a fleet of technologically sophisticated ships, submarines, and aircraft, including 2 aircraft carriers, 2 amphibious transport docks, 4 ballistic missile submarines (which maintain the nuclear deterrent), 6 nuclear fleet submarines, 6 guided missile destroyers, 12 frigates, 11 mine-countermeasure vessels and 26 patrol vessels. As of August 2021, there are 75 operational commissioned ships (including submarines as well as one historic ship, HMS Victory) in the Royal Navy, plus 11 ships of the Royal Fleet Auxiliary (RFA); there are also five Merchant Navy ships available to the RFA under a private finance initiative. The RFA replenishes Royal Navy warships at sea, and augments the Royal Navy's amphibious warfare capabilities through its three Bay-class landing ship vessels. It also works as a force multiplier for the Royal Navy, often doing patrols that frigates used to do.

The Royal Navy is part of Her Majesty's Naval Service, which also includes the Royal Marines. The professional head of the Naval Service is the First Sea Lord who is an admiral and member of the Defence Council of the United Kingdom. The Defence Council delegates management of the Naval Service to the Admiralty Board, chaired by the Secretary of State for Defence. The Royal Navy operates from three bases in Britain where commissioned ships and submarines are based: Portsmouth, Clyde and Devonport, the last being the largest operational naval base in Western Europe, as well as two naval air stations, RNAS Yeovilton and RNAS Culdrose where maritime aircraft are based.