Dalton Castle will not open to visitors for the foreseeable future.
The Grade I-listed 14th century pele tower at Market Place in Dalton-in-Furness is owned by the National Trust.
The Friends of Dalton Castle society released a statement saying they were sorry to announce the news that it would be closed until further notice.
Dalton Castle stands above the town to defend the people of Dalton and the approaches to Furness Abbey. The original castle was possibly built in AD 79.
It is thought the present castle was built between 1314 and 1360, when the county was invaded by the Scots.
After the dissolution of the abbey in 1537, the castle continued as a courthouse for over 300 years, owned at first by the crown and then by the dukes of Albermarle, the Dukes of Montagu, and finally the dukes of Buccleuch.
Its interior has seen many alterations, the last and most radical being that of 1856 when three upper floors, accessible from a stone spiral staircase, were superseded by a single upper room and an additional stairway.
Dalton Castle was given to the National Trust by the 8th Duke of Buccleuch in 1965.
A statement from the society said: “This is a decision taken by the National Trust, and not the friends, who are disappointed with this sudden and unexpected turn of events, especially considering the success that the group has achieved in increasing visitors numbers to the castle over the past couple of years.
“On Tuesday May 14, at the AGM of the friends, National Trust representatives will be present to explain further.”
A National Trust spokesman said: “We have taken the difficult decision to temporarily close the castle so that we can make sure that it’s safe for visitors.
“This period of closure will ensure that we can keep welcoming visitors in the future.”
People can attend the AGM at the castle at 7.30pm and to join Friends of Dalton Castle, the fee is £3 a year.
