HMS J6 was a First World War J-class submarine built for the Royal Navy by HM Dockyard at Devonport in Plymouth.

Under her first commanding officer, Max Horton, J6 was launched on 9 September 1915 and commissioned on 25 January 1916.

Warburton did not identify the fleet as German and did not report his sighting to the Admiralty; had he done so, it is possible that another full scale naval battle may have occurred.

The captain of the Cymric, Lieutenant F Peterson RNR, mistook the identity lettering on the conning tower of J6 for U6.

Assuming U6 to indicate a German U-boat, Peterson raised the White ensign and opened fire on J6.