A previously unknown drawing by Michelangelo which lay unrecognised for more than 250 years, is expected to fetch as much as £8m when it goes up for sale, experts say.

The study of a Mourning Woman was found in a scrapbook during a routine inventory at Castle Howard, in North Yorkshire, about eight years ago.

Since then, various experts around the world have studied the drawing and have confirmed it is a genuine early Michelangelo, dated about 1500.

Now, the work - one of only four in the world still in private hands - is to go up for auction at Sotheby's in London on July 11, when it is expected to sell for between £6m and £8m.

Gregory Rubinstein, director of old master drawings at Sotheby's, said: ''There have been only three major Michelangelo drawings on the market in the past quarter century."

The early drawing was discovered by Sotheby's expert, Julien Stock, while carrying out a routine inventory at Castle Howard - better known as the setting for TV hit, Brideshead Revisited.

While looking through a 19th Century scrapbook in the library at Castle Howard, Mr Stock recognised the work immediately as part of a small group of large-scale figure studies by the Italian Renaissance artist, sculptor and architect.

The study, which shows a three-quarter length view of a woman from the side and with her face obscured, is thought to have languished virtually unseen for more than 250 years.

Although there is no record of when the drawing came to Castle Howard, experts think the drawing was bought by the fourth Earl of Carlisle, Henry Howard, at auction, in 1747.

Only four other similar drawings remain on display - at the British Museum, London, The Louvre in Paris, Munich Print Museum, and Vienna's Albertina.

Michelangelo lived between 1475 and 1564, and was a leading figure in the High Renaissance. He is best known for painting the ceiling of the Sistine Chapel in the Vatican, Rome, and its fresco of The Last Judgement.