A spokesman for Wiltshire Fire and Rescue says that the fire has caused severe damage to the first floor of the three-storey Luckington Manor, which is about 30 per cent of the property.
The Gazette understands that the only occupants in the building at the time were staff and they all left safely.
The blaze is currently contained to one wing of the property, although it has affected both floors and the roof.
Firefighters are currently unable to tackle the flames from within the house, as the structure is unsafe.
Relief crews are now being mobilised to the scene, but the attendance is being scaled back from ten fire engines to six. It is anticipated that the incident will continue for several hours yet.
As yet, there is no suggested cause of the fire – investigations will start as soon as it is safe to do so.