Experts are examining the roof of the Royal Pavilion in Brighton to find out what caused damage to the Music Room.
Part of a decorative feature at the top of the ceiling dropped on to a gasolier below, damaging two of the panels.
The incident occurred when the historic palace was closed on Wednesday night and staff discovered the damage the following morning.
It happened after the wettest June day in Brighton for 36 years and staff think water penetration may have been the cause.
A structural engineer is examining the roof and other experts are looking at what can be done to repair the decorative feature and the gasolier.
Pavilion spokesman Martin Ellis said the piece that fell was not large but caused damage because it fell from a height.
Visitors are still being allowed into the room but there is restricted access during the investigations.
He said: "We have ensured there is no further risk to anyone."
The Music Room has been described as the most beautiful room in Europe and is noted for its ornate decoration.
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