Serco Marine Services is a Private Finance Initiative contract, with Serco Group, to deliver auxiliary services to His Majesty's Naval Service (incl. Royal Navy, Royal Marines and Royal Fleet Auxiliary).
Marine Services primarily operates from the nation's three main naval bases, HMNB Portsmouth, HMNB Devonport and HMNB Clyde, but also supports training and operations overseas, as well as at various British Overseas Territories.
His Majesty's Naval Service, referred to colloquially as the Royal Navy, after the primary fighting arm of the service, is the United Kingdom's naval warfare and maritime service. It consists of the Royal Navy, Royal Marines, Royal Fleet Auxiliary, Royal Naval Reserve, Royal Marines Reserve and Naval Careers Service. The Naval Service as a whole falls under the command of the Navy Board, which is headed by the First Sea Lord. This position is currently held by General Sir Gwyn Jenkins (appointed May 2025). The Defence Council delegates administration of the Naval Service to the Admiralty Board, chaired by the Secretary of State for Defence.
The Naval Service is dominated by the Royal Navy, and operates primarily from three bases in the United Kingdom where commissioned ships are based; Portsmouth, Clyde and Devonport, the last being the largest operational naval base in Western Europe. As of December 2025, there were about 90 vessels in service with the various branches of the Naval Service, plus more than 90 additional vessels operated by the supporting Serco Marine Services. These vessels included: 63 commissioned ships of the Royal Navy, 17 principal landing craft of the Royal Marines, 10 auxiliary ships of the Royal Fleet Auxiliary and 90+ supporting Marine Services vessels. Additional vessels are under charter to the Ministry of Defence. As of 2022, HM Naval Service (Royal Navy, Royal Marines, Royal Fleet Auxiliary and Naval Careers Service) employed about 39,500 regular and reserve personnel.