Some 80 fire-fighters from 16 crews are tackling a huge blaze at the National Trust-run stately home of Clandon Park House near Guildford in Surrey.
According to eye-witnesses the fire, which apparently started in the basement at around 16.00 BST, soon spread to the roof and apparently gutting much of the interior.
Photographs circulating on social media appear to show huge flames bursting from the building and thick smoke rising from the blaze. Fire crews are wearing breathing apparatus and using long ladders and water carriers as they attempt to quench the flames.
A spokesman for Surrey Fire and Rescue Service said: "This afternoon at 4.09pm we were called to a fire at Clandon Park, West Clandon, Surrey, a stately home owned and managed by the National Trust.
"On arrival, firefighters were faced with a significant fire throughout the building which is believed to have started in the basement and quickly spread to the roof.
"Everyone was evacuated from the building and firefighters are now tackling the blaze as well as seeking to save artefacts from the property. In total, sixteen fire engines from Surrey, West Sussex and Hampshire, including two aerial ladder platforms, a number of water carriers and specialist appliances have been at the scene.
"The operation is expected to continue throughout the evening, with fire investigation officers on scene looking to establish the cause. There are currently no reports of injuries."
One witness, Hayley Bystram, tweeted: "So sad to see beautiful @ClandonParkNT go up in flames Such a stunning piece of history and architecture #clandonpark"
Another witness Chris Nicholson posted: "The smell of burning and the thick smoke is continuing and we have to close the windows."
According to the National Trust, Clandon Park was built by a Venetian architect for Lord Onslow in the 1720s and is one of the most complete Palladian Mansions in the country. The house was donated to the Trust in 1956 and is used as a venue for weddings and corporate functions.