Hurst Castle is facing huge and unexpected erosion problems.
Castle manager Sean Crane says that there had been a dramatic change in the depth of the water on the sea side of the castle.
There has been an emergency meeting this week between English Heritage who maintain the castle and New Forest District Council.
The council added more than 600,000 tonnes of gravel to Hurst Spit on 1996.
Friends of Hurst Castle chairman David Jupp says no one seems to know why it has happened.
English Heritage told the New Milton Advertiser: “We are concerned about coastal erosion affecting the foundations of Hurst Castle.
“Coastal protection works were undertaken around 20 years ago and these had been effective until relatively recently.
“However, this winter has seen an area of erosion develop along the east wing where the level of shingle along the beach has dropped in some areas. This has resulted in part of the concrete foundations now being exposed to high tides.”