A little-known work by Italian artist Titian is expected to fetch more than £5m when it is auctioned in December.
The painting - Portrait of a Lady and her Daughter - probably dates from the 1550s but was painted over in the late 16th or early 17th Century.
It was only recently revealed after nearly 20 years of careful restoration.
"This remarkable and moving portrait is one of the most notable discoveries of recent years," said Francis Russell of Christie's auction house.
"Titian was one of the greatest and most influential of all portrait painters."
Thought to depict his daughter Emilia with one of his granddaughters, the uncompleted portrait is the only known work by Titian to show a mother and daughter.
The canvas was reworked after Titian's death, presumably for commercial reasons, as a portrait of the Archangel Raphael with the boy Tobias.
The painting beneath was spotted in 1948 and led to a lengthy restoration process.
It was subsequently exhibited at Kenwood House, London, at the end of 2001 and formed part of the Madrid leg of 2003's major Titian exhibition.
Titian (Tiziano Vecellio) is widely regarded as one of the most pioneering of all Renaissance artists.
Another of his masterpieces, Portrait of a Young Man, is set to fall into private hands after a plan to purchase it for the nation fell through.