HM Submarine X1 was a British submarine of the interwar period. Conceived and designed as a submersible commerce raider for the Royal Navy; at the time of her launching she was the largest submarine in the world. For Britain, the idea of a submarine cruiser had been proposed as early as 1915, but the type was not put into practice after the end of World War I in 1918. X1 was laid down on 2 November 1921 at HM Dockyard, Chatham and completed on 23 September 1925 with commissioning following in December 1925.