A fire that destroyed an 18th Century mansion started accidentally and was probably caused by an electrical fault, investigators have said.
Clandon Park House near Guildford, which housed a collection of furniture, porcelain and textiles, was reduced to a charred shell during the blaze.
A fire report said it was most likely the result of a faulty connection in the electrical distribution board.
The National Trust said a full review of fire prevention policies had begun.
A lack of fire protection to the fuse cupboard ceiling and the stately home's historic design allowed the fire to spread, the Surrey Fire and Rescue Service report said.
Such buildings are adapted over the years to add amenities, producing hidden voids that may or may not be known because of the loss of buildings plans and records, it added.
'Checks on all mansions'
"It is the hidden voids in this type of building that allow unpredictable and uncontrollable fire to spread," it said.
Assistant chief fire officer Simon Moore said the National Trust had been advised of the findings.
National Trust director general Helen Ghosh said the fire had come as a "terrible blow".
Staff evacuated the house safely and no-one was injured. A salvage plan helped to save contents and fire detection systems operated as they should have done, she said.
"We're certainly not complacent and we now will be working with the fire service to identify any areas for improvement in any of our properties."
She said a full review of fire prevention policies had begun, including checks on distribution boards at all the trust's historic mansions.
The trust said none of its staff would have been able to identify the fault as a potential issue, and it had not been detected during professional checks by electricians.
Mr Moore said the fire service had begun work with the trust to consider what more could be done to improve the fire safety of their other buildings.
Fire broke out at the Grade-I listed building in April, reducing it to a shell.
One room - the Speakers' Parlour - survived and hundreds of items have been rescued.
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