- Femme endormie will be sold on July 28 with an estimate of £6million-£9million
- The charcoal drawing was created in 1931 and stayed with Picasso until his death
- Picasso was 45 years old when he fell in love with Walter who was 17 at the time
A portrait by Picasso of his lover Marie-Thérèse Walter is set to appear at auction for the first time.
Sotheby's will auction the charcoal drawing created in 1931 which the famous Spanish artist personally kept until he died in 1973.
The largely unseen piece of art shines a light on one of the greatest love affairs of the 20th century and was drawn when the romance was still very much a secret.
The Picasso, titled Femme endormie, will be sold on July 28 and has an estimate of £6million-£9million.
Helena Newman, worldwide head of Sotheby's impressionist and modern art department, told The Guardian: 'It is an absolute gem, it is so exquisitely beautiful. He has applied a technique which gives an incredible intimacy to the work.
'It's charcoal and is effectively the hand literally almost caressing her. It is just wonderful, it looks back to the great drawings of the Renaissance yet it is so incredibly modern and free and spontaneous and direct.
'It is like you are looking over his shoulder as he is drawing it.'
Picasso was 45 years old when he fell in love with Walter who was 17 at the time.
They met outside a department store in Paris in 1927, when he promised her they would 'do great things together'.
They conducted their affair in secret for several years until his wife, Olga Khokhlova, found out and left in disgust.
His grandson Olivier Widmaier Picasso said the affair was like Picasso being 'reborn'.
Walter inspired several of his paintings, drawings and sculptures, some of which critics say are the greatest works of his career.
Many of these pieces were featured in a 2018 exhibition at the Tate Modern.
The portrait going on sale was bought by its present owner in 1986 and has not been seen nor publicly exhibited since.
Other Picasso paintings from 1932 that were inspired by Walter have often sold for millions.
In 2010, Nude, Green Leaves and Bust sold for £66million while Le Rêve was bought for £103million by hedge fund manager Steven A Cohen.
Femme endormie forms part of a private collection of works that tells the story of 20th century European avant-garde art.
The collection includes work from artists like Wassily Kandinsky, Lyonel Feininger, Alberto Giacometti, Marc Chagall, August Macke and Jean Arp.