No one was hurt in the fire, which is thought to have started in the basement, but parts of the building have been left gutted.
Dame Helen Ghosh, director general of the National Trust, which manages Clandon Park in Surrey, said: "The fire is now out but the scale of the damage to the mansion has been devastating. The house is now essentially a shell, most of the roof, ceiling and floors have collapsed into the bottom of the building. There is perhaps one room that is relatively untouched, but other than that the interior is extensively damaged. The external walls are still standing."
At the moment it is unknown what will happen to the property, but functions and weddings due to happen in the near future have been cancelled, with a message on the venues website stating that it would be contacting anyone who had booked a function there by 2 May.
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