Part of Butleigh Hospital has been destroyed in a suspected arson attack overnight.
Dozens of firefighters have braved high winds throughout the night to try to halt the spread of the blaze at the rural site near Glastonbury.
First reports of the fire were made after midnight when flames were spotted in the west wing of the derelict building, which closed in 2005.
Specialist units from across the county were called in as conditions worsened and the fire spread.
By 5.30am the fire was well under control and many exhausted firefighters were allowed home.
Officers are now awaiting for daylight for a full inspection but already say they believe the once much-loved facility was deliberately set alight.
At least a quarter of the historic building has been destroyed.