The Stickleback class submarines were midget submarine

Midget submarine

A midget submarine is any submarine under 150 tons, typically operated by a crew of one or two but sometimes up to 6 or 8, with little or no on-board living accommodation...

s of the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 initially ordered as improved versions of the older XE class submarine

XE class submarine

Six XE-class midget submarines were built for the Royal Navy during 1944. They were an improved version of the X Class midgets used in the attack on the German battleship Tirpitz....

s. They were designed to allow British defences to practice defending against midget submarines since it was theorised that the Soviets had or could develop such craft.

The Royal Navy developed plans to use these craft to carry a 15 kiloton nuclear mine (based around the Red Beard weapon

Red Beard (nuclear weapon)

Red Beard was the first British tactical nuclear weapon. It was carried by the English Electric Canberra and the V bombers of the Royal Air Force, and by the Blackburn Buccaneers, Sea Vixens and Supermarine Scimitars of the Royal Navy's Fleet Air Arm...

) codenamed Cudgel into Soviet harbours. The project was unsuccessful as there were problems finding and paying for the necessary fissile material.

Boats

There were 4 boats, launched 1954–1955:

• X51 Stickleback, launched July 1954, sold to the Royal Swedish Navy in 1958 and was renamed Spiggen. Now on display at the Imperial War Museum Duxford
  Imperial War Museum Duxford
  Imperial War Museum Duxford is a branch of the Imperial War Museum near the village of Duxford in Cambridgeshire, England. Britain's largest aviation museum, Duxford houses the museum's large exhibits, including nearly 200 aircraft, military vehicles, artillery and minor naval vessels in seven...
  
  .
• X52 Shrimp, launched October 1954
• X53 Sprat, launched 30 December 1954
• X54 Minnow, launched 5 May 1955