The wreck of an early British submarine off the coast of Devon has been given protected status.
The wreck, which sits upright and largely intact on the seabed, was identified by divers capable of working at depths of more than 40 metres.
It has now been granted protection by the Department for Digital, Culture, Media and Sport on the advice of Historic England.
This means divers can still dive the wreck, but its contents are protected by law and must remain in situ.
Lead diver Steve Mortimer said the team expected to find the wreckage of a German U-boat, but were "thrilled to discover a ground-breaking British submarine instead".
"Every diver dreams of identifying a historically important wreck," he said.
"It's tremendous that D1 is now protected but divers can still visit."
Launched in 1908 and commissioned in 1909, the vessel was also the secret prototype for the Navy's D-Class submarine – the service's first diesel-powered submarines.
At the beginning of the war in 1914, the submarine was assigned to protect the coast of Dover from enemy invasion and in 1917, the HMS/m D1 joined the Portsmouth local defence flotilla.
A year later it was relegated to training duties and, weeks before the end of the war, it was decommissioned and scuttled off the Dartmouth coast.
The submarine was later used as a target for Royal Navy training exercises involving the detection of enemy submarines.
Duncan Wilson, chief executive of Historic England, said the D-class submarine had "innovations that became integral" to future Navy submarines.
"These included diesel propulsion, twin propellers and a wireless telegraphy system which allowed the submarine to transmit and receive signals," he said.
"This is a fascinating survival which deserves protection as an important part of our seafaring history."
Eight D-class submarines were built.
HMS/m D2, HMS/m D3 and HMS/m D6 were sunk outside English territorial waters, while HMS/m D4, HMS/m D7 and HMS/m D8 were sold and scrapped in 1919.
The wreck of HMS/m D5 is located off Lowestoft in Suffolk, and is protected under the Protection of Military Remains Act 1986.