Hurst Castle and Calshot Castle in Hampshire are both at risk of being lost forever because of rising sea levels, according to English Heritage.
The charity fears for the future of several heritage sites it manages in the UK and has launched a multimillion-pound fundraising appeal to fund works to halt the damage.
Hurst Castle, an artillery fortress built by Henry VIII, saw a huge section of the 18th century east wing collapse in February 2021 after the sea undercut its foundations.
English Heritage said work to stabilise the damaged section has been completed, but warned the sea walls around the original Tudor fort are also in urgent need of repair at an estimated cost of £160,000.
Just down the coast, Calshot Castle, another of Henry VIII's fortifications, is battling erosion, but its low lying site also puts it at risk from flooding as sea levels rise.
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 millenia.
Rob Woodside, Director of Estates at English Heritage, said: "Hundreds of heritage sites in the UK and around the world are increasingly at risk.
"If these coastal properties are to survive the coming decades, we will need to strengthen their walls and build sea defences to protect them. It is for this reason that we are launching a public appeal to raise funds for this vital conservation work."