LONDON (REUTERS).- The family of Princess Diana will auction a painting by Flemish master Peter Paul Rubens as part of a clearout sale valued at $30 million designed to ensure the financial future of the Althorp estate.

Earl Spencer, Diana's brother, is head of the estate, and plans to marry for a third time later this year, according to newspaper reports. Diana, who was married to heir-to-the-throne Prince Charles, was killed in a car crash in Paris in 1997.

The highlights of a collection including paintings, furniture, porcelain and horse-drawn carriages are works by Rubens and 17th century Italian master Il Guercino.

The Rubens, "A Commander being armed for Battle," was painted in 1613 or 1614 and is expected to fetch 8-12 million pounds ($12-18 million). The bearded man gazing toward the viewer has been named by some experts as Emperor Charles V. The painting has been at the late princess's family home Althorp for more than 200 years, and was originally described as the "school of Rubens" in the 1802 inventory. Since then it has been recognized as a Rubens original by leading scholars, Christie's said.

"This magnificent work by Rubens is one of the most important works by the artist to remain in private hands in the UK," said Richard Knight, co-international head of old masters and 19th century art at Christie's.

"It is a museum quality masterpiece and will have wide international appeal."

Another work by Rubens, "The Massacre of the Innocents," sold for 49.5 million pounds at Sotheby's in London in 2002, a world auction record for an old master.

Also being sold by the Spencer family is "King David" by Baroque artist Il Guercino, which was bought to hang in the Great Room at Spencer House in London.

It is expected to raise 5-8 million pounds when both works go under the hammer as part of Christie's old masters sale in London on July 6.

Althorp is undergoing a 10 million restoration, and the trustees of the estate said they had decided to sell items which were "not core" to the collection.

"Their aim is to help the Althorp Estate to thrive for generations to come," the trustees said in a statement.

Christie's main rival Sotheby's also announced an important sale last week, with J.M.W. Turner's "Modern Rome - Campo Vaccino" set to raise 12-18 million pounds at the auctioneer's London sale on July 7.


(Reporting by Mike Collett-White, editing by Paul Casciato)