Part of the east gantry at the historic Hurst Castle collapsed into the sea on Friday (February 26).
The castle can be clearly seen from the West Wight coastline and it is the starting point for the annual West Wight Solent Swim.
English Heritage, who own the historic building, said: “Everyone’s safety is our first priority and there are no reports of injuries. The Castle was closed.
“It’s a terrible blow for this great site and all who care for it. Our engineering team is assessing the damage and next steps.”
Repairs were already being carried out on parts of the Castle’s east wing where it had been undermined by coastal erosion.
English Heritage had already completed extensive work to the west wing battery after it was hit by storms and high waves in November 2019. Broken groynes and barriers were replaced while the beach was replenished with 7,500 tonnes of shingle. The foundations were also underpinned.
Built by Henry VIII to counter the threat of French aggression, the 573-year-old structure also helped defend Britain from the Nazis in the Second World War.