The Royal Naval Auxiliary Service (RNXS) was a uniformed, unarmed, civilian volunteer service, administered and trained by the Royal Navy to operate in the ports and anchorages of the United Kingdom in an emergency. Although the abbreviated title would logically have been RNAS this abbreviation had long been taken by the various Royal Naval Air Stations, so RNXS it was. It maintained training units, and vessels at most major ports in the UK. and was formed in 1963 from the amalgamation of the Royal Naval Mine-watching Service (RNMWS) and Admiralty Ferry Crew Association in response to the perceived nuclear threat to British ports. The service was disbanded on 31 March 1994 due to Ministry of Defence (MOD) cuts. Most vessels from its fleet were transferred to the Royal Navy or sold, with the exception of XSV Loyal Volunteer, which was struck by a ro-ro ferry while berthed in Ipswich Harbour and was later scrapped.

The Royal Naval Mine Watching Service was a civilian organisation formed in 1952 as part of the auxiliary forces of the British Naval Service.