The Battle of May Island is the name given to the series of accidents that occurred during Operation E.C.1 in 1918. Named after the Isle of May, a nearby island in the Firth of Forth, the "battle" consisted of a disastrous series of accidents amongst Royal Navy vessels on their way from Rosyth, Scotland, to fleet exercises in the North Sea. On the misty night of 31 January–1 February 1918, five collisions occurred between eight vessels. Two K-class submarines were lost and four other submarines and a light cruiser were damaged. 105 British sailors in total died in the accidents.

The Isle of May is located in the north of the outer Firth of Forth, approximately 8 km (5.0 mi) off the coast of mainland Scotland. It is about 1.5 kilometres long and 0.5 kilometres wide. The island is owned and managed by NatureScot as a national nature reserve. There are now no permanent residents, but the island was the site of St Adrian's Priory during the Middle Ages.