Earl Spencer's estate is set to receive a £20 million windfall from a sale of Old Masters paintings and other treasures owned by the family, it was announced.
Among the items to be auctioned from Althorp, the ancestral home of the late Diana, Princess of Wales and her brother, is a painting by baroque artist Sir Peter Paul Rubens.
The work, A Commander Being Armed for Battle and painted in about 1613-14, is expected to fetch up to £12 million when it goes under the hammer at auctioneer Christie's in London on July 6.
A second painting, King David, by 17th century Italian artist Giovanni Francesco Barbieri, is expected to sell for as much as £8 million.
Althorp, in Northamptonshire, is undergoing a £10 million reroofing and restoration project.
The auction of artwork, English and French furniture and porcelain from the Spencer Collection and treasures from Althorp's attics will fund reinvestment in Althorp and its estate.
A group of 19th century horse-drawn carriages from the Spencer Carriage Collection and items from Spencer House, the family's London town house until 1924, will also go under the hammer.
The Trustees of Althorp Estate said in a statement: "For 18 years, the Trustees at Althorp have significantly built up the estate through the acquisition, development and retention of commercial and residential property.
"The Trustees believe now is the time to prepare for further, positive diversification and have therefore elected to sell a selection of works of art that are not core to the Althorp Collection.
"Their aim is to help the Althorp Estate to thrive for generations to come."
Orlando Rock, director of Christie's collections department, said the sale offered international collectors an unprecedented opportunity to acquire artworks from one of the most historic English collections.
He said: "Like so many of the great aristocratic treasure houses, Althorp has, over the centuries, become the repository for works of art and furniture from numerous family houses, many of which have long disappeared or been sold.
"The rich and varied sales this summer have therefore been carefully curated to ensure that the historic integrity of the 18th century collections at Althorp remain intact, focusing instead upon later additions by both inheritance and purchase."
Highlights of the Spencer House sale to be held on July 8 include an array of English seat furniture - George III gilt wood open armchairs, a George II mahogany sofa, George II mahogany open armchairs and George II gilt wood stools - all expected to fetch six-figure sums.
Experts are still working from room to room in Althorp's labyrinth of attics, cellars and stables, discovering a trove of artworks, ceramics, silver, textiles, furniture, pictures and military articles.
More than 500 lots are destined for the Spencer Carriages and Attic Sale, also to be held this summer.
The Althorp Estate covers 14,000 acres of countryside in Northamptonshire, Warwickshire and Norfolk.
This announcement follows news last week that Earl Spencer, 45, is to marry for the third time.
He will wed Lady Bianca Eliot, 33, possibly as early as September.
The couple have been dating for the past 18 months.