Several galleries at York Castle Museum have been closed following the discovery of reinforced autoclaved aerated concrete (RAAC) in the building's roof.
The museum is on the site of the city's former prison, constructed between 1780 and 1783. Part of it was first converted into a museum in 1938, and the roof has undergone substantial repairs over the years.
The museum's Kirkgate exhibition as well as the Shaping the Body and Period Rooms displays have been closed while further inspections take place.
Kathryn Blacker, Chief Executive of York Museums Trust, said the closure was a precautionary measure.
"We're doing all we can to ensure the necessary inspections take place as soon as possible and are planning for any potential actions we will need to put place when we receive the results of the inspection," she said.
The rest of the museum is to remain open with ticket prices being reduced during the closure.