The Stickleback-class submarines were midget submarines of the British Royal Navy initially ordered as improved versions of the older XE-class submarines. They were designed to allow British defences to practice defending against midget submarines since it was theorised that the Soviet Union had or could develop such craft.
The Royal Navy developed plans to use these craft to carry a 15-kiloton nuclear naval mine (based on the Red Beard weapon) codenamed Cudgel into Soviet harbours. The project was unsuccessful as there were problems finding and paying for the necessary fissile material.
There were four boats, launched 1954–1955:
X51 Stickleback, launched 1 October 1954, 12:30hrs, sold to the Royal Swedish Navy in 1958 and was renamed Spiggen (Swedish name for "Stickleback"). Sweden decommissioned her in 1970 and after a few years returned to the UK. After a period on display at the Imperial War Museum Duxford, and then in storage at Portsmouth Naval Dockyard, X51 was moved to Faslane in September 2016 in preparation for exhibition at the new Scottish Submarine Museum at Helensburgh.
X52 Shrimp, launched October 1954, scrapped 1965
X53 Sprat, launched 30 December 1954, loaned to US Navy 1958, scrapped 1966
X54 Minnow, launched 5 May 1955, scrapped 1966