Picasso's portrait of his mistress to fetch £29m

One of Pablo Picasso's largest and most important portraits of his long-time mistress and artistic inspiration, Dora Maar, is to be auctioned at Sotheby's in New York this spring.

The auction house said yesterday that it expected to draw bids of as much as $50m (£29m).

The three-quarter length portrait, Dora Maar with Cat, was described by Charles Moffett, the vice-chairman of Sotheby's, as "unquestionably one of Picasso's most extraordinary portrayals of the woman who for nearly a decade was his muse, model and lover". It has not been seen publicly for 40 years.

The picture, painted in 1941 and measuring 50x37in (127x94cm), will be put under the gavel on 3 May in New York as part of the auction house's annual Impressionists and Modern Art sale. It depicts his lover seated, her head characteristically adorned with an extravagant hat with feathers, with a small black cat looking on.

Scholars of Picasso have long credited Dora Maar, who was herself a well-regarded photographer, with inspiring Picasso during one of his most turbulent and creative periods when first the civil war wrackedhis native Spain and then the Second World War broke out in Europe.

The announcement by Sotheby's comes a day after the opening of a widely praised exhibition dedicated to the couple's creative relationship at the Picasso Museum in Paris. The exhibition, which runs until 22 May, offers by far the most extensive collection of works by both Picasso and Maar completed during their eight-year relationship, which ended in 1945 when Picasso fell for another woman.

According to legend, the love affair began when Picasso spotted Maar at Les Deux Magots, the café on the Left Bank in Paris that was later made famous as the haunt of Jean-Paul Sartre and Simone de Beauvoir. He watched as she took a penknife and stabbed it between her fingers on the table, occasionally wounding her hands and causing blood to seep into her gloves.

Picasso followed her out of the café and thus began his love affair with a woman who was close to several key figures of the surrealist circle in Paris. She had a wild beauty Picasso described as feline. She took countless photographs of him, many now on display in Paris. Among the paintings she helped Picasso to realise was Guernica.

"Dora Maar helped Picasso create a body of work that lived up to the times," Anne Baldassari, the director of the Picasso Museum, said. "Perhaps he needed a woman who was as complex and as sad as Dora Maar to reach this level of expression."