HMS K26 was the only modified K-class submarine of Britain's Royal Navy to be completed.

Six boats, K23-K28 were ordered in June 1918 but the end of the war meant that only K26 was completed, the remainder all being cancelled on 26 November 1918.

Although launched at Vickers yard in August 1919 she was towed to Chatham in 1920 and completed in June 1923.

Improved ballast tank arrangements cut the diving time to 3 minutes 12 seconds to get to 80 feet (24 m).

In 1924 she embarked with much publicity on a long voyage via Gibraltar, Malta and the Suez Canal, to Colombo and Singapore and back again.