Part of the wall of a castle, originally built in Tudor times by Henry VIII, has collapsed.
A section of Hurst Castle's east wing, near Milford-on-Sea, Lymington, fell on Friday.
Owner English Heritage said the sea had weakened the foundations. It added engineers were assessing the damage and planning remedial works.
The site was closed to the public at the time of the collapse and there have been no reports of any injuries.
Most recently, English Heritage carried out work in 2019 to stabilise the foundations of the west wing and to reinforce its sea defences.
It confirmed a programme of works to protect the 19th Century east wing was "about to commence".
Estates director Rob Woodside said: "This is a devastating blow to a Hampshire icon and for all of us whose life's work is to protect England's historic buildings."
The organisation said it was among "the most challenging and most difficult heritage to protect" due to its "extremely vulnerable position, exposed to the full onslaught of the sea".
"Located on a shingle spit the castle faces the full force of the wind and waves," it added.
Hurst Castle was originally built by Tudor king Henry VIII between 1541 and 1544 to guard Needles Passage, which is the narrow western entrance between the Isle of Wight and the mainland.
From 1860, the large east and west wing batteries were added as part of a defence programme.
The castle was also used for searchlights and guns during World War One and World War Two.