Works of art, furniture, and pictures, valued at pounds 15m, from Houghton Hall, Norfolk, are to go under the hammer on 8 December. Nothing comparable has been seen since the Duke of Devonshire sold a group of Old Master drawings from Chatsworth in 1984 for pounds 21m.
Houghton, a sumptuous Palladian pile, was built by Britain's first prime minister, Sir Robert Walpole between 1722 and 1735.
While Walpole's dissolute nephew sold the Houghton picture collection to Catherine the Great in 1791, none of Walpole's original furnishings have ever previously left the house.
The sale is an attempt by the 7th Marquess of Cholomondeley to restructure a heritage deal on Houghton's treasures, which came unstuck in 1992.
The furniture includes eight pairs of chairs from the very large sets ordered by Walpole for his reception rooms. They are all elaborately carved and gilded, and are still upholstered in their original green velvet. Nothing comes from the rooms currently open to the public.
All the chairs the marquess has selected for sale have been mouldering in storage since around 1800 when they were moved from their original positions in the house.
Despite dilapidation, they are expected to fetch up to pounds 200,000 a pair. He is also offering some plain chairs which Walpole bought for the bedrooms.
In addition, the marquess is selling the greatest treasures from the collection formed by his great uncle, Sir Philip Sassoon, who was chairman of the trustees of the National Gallery between the wars. They comprise superb French furniture, 18th century ormolu-mounted porcelains of royal magnificence, two ormolu swans which are believed to have been made for Madame de Pompadour, Louis XV's famous mistress, and a group of paintings which includes La Lecture de Moliere by Jean-Francois de Troy, estimated at pounds 3m- pounds 5m and a double- sided oil sketch by Rubens, estimated at pounds 1.5m- pounds 2.5m.
The 33-year-old marquess inherited Houghton from his grandmother in January 1990. Then, two months later, his father died leaving him the 9,000- acre Cholmondeley estate in Cheshire, which came with 40 tenant farmers.
His sisters did not share the inheritance and, ever since, the marquess has been looking for a tax-effective way to raise money to share between the family and to pay for urgent repairs at Houghton. His aim has been to raise pounds 20m after tax.
Holbein's Lady with a Squirrel - originally destined for Christie's - has already been sold to the National Gallery for pounds 10m. But negotiations to sell Walpole's furniture to the National Heritage Memorial Fund came to nothing.