The fire could "easily have been prevented", the fire service said
A Grade II-listed stately home in north Dorset is in danger of collapsing after a fire overnight.
Up to 100 firefighters tackled the blaze at Holnest Park House south of Sherborne. No one was hurt.
It was thought to have been started by an electric blanket in a flat on the ground floor, a fire service spokeswoman said.
She said the centre of the house was declared structurally unsafe and the ceiling was in danger of collapsing.
The flames spread through the centre of the building, which is divided into seven flats, but crews managed to stop it from spreading to either wings.
The residents had already evacuated the building when crews arrived shortly after 1900 GMT on Saturday.
Crews remained at the scene overnight and the fire flared up again in the early hours, but has now been put out.
An investigation found the most likely cause was an electric blanket, left on for "some hours".
Mike Cox, fire investigation officer and group manager, said: "Although no-one was injured in this incident, a substantial amount of damage has been caused to the centre two flats of this imposing property.
"Some very, very simple safety measures would have prevented this happening.
"Electric blankets are perfectly safe to use so long as they are not damaged, are thermostatically controlled, are tested regularly by qualified electricians and are stored correctly, either flat or rolled, not folded.
"In this case the blanket was left on without a thermostat so acted as an unmonitored heating device which did just that, continued to heat until it caught fire."
The mansion dates back to the 18th Century but was converted into separate flats in the modern era.