The original pair of battleships became the Colossus class and were improved versions of the preceding battleship, HMS Neptune. The initial 1909–1910 Naval Programme included two dreadnoughts and a battlecruiser, but was later increased to six dreadnoughts and two battlecruisers as a result of public pressure on the government due to the Anglo-German naval arms race. While being towed to the scrapyard, the ship ran aground Thunderer was refloated and subsequently broken up. Thunderer served the longest, acting as a training ship from 1921 until she, too, was sold for scrap in late 1926. In late 1923, she was converted into a target ship and was sunk in early 1925. Orion and Conqueror were sold for scrap in 1922 while Monarch was hulked for use as a stationary training ship.

The Orions were deemed obsolete by the end of the war in 1918 and were reduced to reserve the following year. Aside from participating in the failed attempt to intercept the German ships that had bombarded Scarborough, Hartlepool and Whitby in late 1914, the Battle of Jutland in May 1916 and the inconclusive action of 19 August, their service during World War I generally consisted of routine patrols and training in the North Sea. The sister ships spent most of their careers assigned to the 2nd Battle Squadron of the Home and Grand Fleets, sometimes serving as flagships. The first 13.5-inch-gunned (343 mm) battleships built for the RN, they were much larger than the preceding British dreadnoughts and were sometimes termed ' super-dreadnoughts'. The Orion-class battleships were a group of four dreadnought battleships built for the Royal Navy (RN) in the early 1910s.

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