Tintagel Castle in Cornwall, famous in British mythology as the birthplace of King Arthur, is now at risk of being destroyed by coastal erosion as sea levels continue to rise.
The Cornish castle is one of several sites at risk of being lost forever, English Heritage has warned.
Rob Woodside, director of estates at English Heritage, said: "Erosion along England’s coastline is nothing new but the rate of land loss that we have seen over the past few years is alarming, and some scenarios indicate that sea levels could increase by up to a metre by the end of the century."
The heritage body described the rate of land lost over the last few years as "alarming", warning that sea levels are now rising at their fastest rate for nearly three millennia.
"Last century sea levels rose by 14cm along the southern coast of England. Climate change is creating huge challenges to protect it," he added.
Tintagel Castle was built in the late Roman period, but it was not until the 12th century that chronicler Geoffrey of Monmoth claimed it to be the birthplace of King Arthur.
English Heritage said the site has always battled with erosion, with parts of the castle having already fallen into the sea by the 14th century.
But recently parts of the cliff directly in front of the visitors centre has been lost, affecting the viewing area and the coastal path.
Other castles considered to be among the most vulnerable to coastal erosion include Bayard’s Cove Fort near Dartmouth in Devon, which was built in Tudor times to defend the entrance to the Dart Estuary.
"If these coastal properties are to survive the coming decades, we will need to strengthen their walls and build sea defences to protect them."