Senior NUT members said they were "devastated" at news of the blaze at Stoke Rochford Hall, near Grantham, Lincolnshire, which has doubled as a training base and conference centre since the union bought it 27 years ago.
Staff spotted smoke coming from under the door of a top-floor archive room just before 9am on Tuesday. The early warning gave them a chance to evacuate the Grade I listed building and, with the help of delegates on a Customs and Excise course, rescue some of its most valuable art treasures.
Although, the Lincolnshire fire service was on the scene within minutes, it was unable to prevent the blaze rapidly taking hold. By lunchtime all the glass at the front of the building had been blown out. The roof had collapsed and fallen through two floors, devastating the oak-panelled Grand Hall.
Function rooms off the hall, including the library, were also damaged. But the ceiling protecting the Oak Room which contains a magnificent black and white marble fireplace survived. A fire service spokesman said there had been "severe damage" to more than half the building.
The cause is still being investigated. An NUT spokeswoman said it was too early to estimate the cost of the damage.
Stoke Rochford, set in 28 acres of formal gardens surrounded by 1,000 acres of parkland, has 31 meeting rooms, the largest of which seats 500 delegates. Conference packages including three meals and an en-suite bedroom are priced cost at between pound;78-pound;130 per person. Income from outside conferences is used to subsidise the NUT's own training courses.
Peter Robinson, director of Stoke Rochford since 1979, said: "This is a terrible tragedy. At the moment I am feeling somewhat numb to see such a magnificent place damaged so badly."
Designed by Scottish architect William Burn, the hall was built between 1840-5 for Sir Christopher Turnor, a local landowner whose family still hold the freehold. During the Second World War it was the HQ for the second battalion of the Parachute Regiment and afterwards, Kesteven teacher-training college which closed in the 70s.
In 1978 the NUT bought the hall on a 1,000-year lease and has used it for its training courses as well as developing it into a commercial residential conference centre.
Former general secretary Doug McAvoy helped to persuade the union to purchase the hall and rushed down to Stoke Rochford when told of the fire this week. He said: "I just feel shattered. It has been part of my life for nearly the entire time I was at the NUT."