A landslip under the oldest part of Cockermouth Castle has left it exposed.
Calls have been made for the situation to be assessed immediately.
The floods triggered the landslide which washed away all the vegetation under the ancient structure which overlooks the River Derwent.
Phil Campbell, chairman of Cockermouth and District Civic Trust, said: "It's very worrying. As soon I was made aware of it I immediately informed Lord and Lady Egremont. I had a quick reply from them just to say that they are aware of the situation.
"I also informed Historic England as the castle is a grade one listed building and clearly 'at risk'.
"I'm worried that any repair will be very difficult given the steep location above the river. The slip hasn't exposed any supporting rock structure."
Darren Ward, the civic trust's architectural adviser, has informed Allerdale council's building controller.
He said: "What concerns me is there is no evidence of rock under there, it has no structural integrity. All the vegetation has gone and the land is slipping away."
A spokesperson for Leconfield Estates, which owns Cockermouth Castle, said: "We are aware of the damage at Cockermouth Castle caused by the recent storms and flooding.
"We are assessing the damage and propose to undertake appropriate remedial works in due course when weather and ground conditions allow."
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