'Insignificant' painting left for 150 years in National's vaults is £10m Botticelli

A masterpiece by Sandro Botticelli will go on public view next month after languishing in the vaults of the National Gallery for more than 150 years.

St Francis with Musical Angels, bought from an art dealer in the mid-19th century, is now believed to be worth up to £10m.

But not until the painting was cleaned recently did the National realise it had been wrongly attributed to one of the Italian Renaissance artist's followers. It was seen as insignificant.

This week, experts examined the masterpiece further using infrared reflectography. "There was a very nice drawing – a linear outline of St Francis – underneath which confirmed the attribution," Carol Plazzotta, curator of Italian Renaissance, said.

It was painted 10 years before Botticelli's most important works, including his celebrated The Birth of Venus with the goddess rising from a shell.

St Francis was bought by the first director of the National Gallery, Sir Charles Lock Eastlake, in 1858 from a dealer in Ferrara, but has remained in storage since.

"The little angels around St Francis have emerged very well in the cleaning," Ms Plazzotta said. "Their heart-shaped faces gave the first clue to it being an early work by Botticelli. There is incredible detail in the painting; for example the way St Francis's tiny crucifix has little drops of blood from the wounds of Christ. The angels have different instruments and the strings of the harp are in gold. It is has got an exquisite quality. It really shines in certain lights."

The painting, missed because of an unflattering frame and gilded background, was identified by Jill Dunkerton, a restorer at the gallery. "It became immediately apparent there were passages of beautiful quality painting, identical to Botticelli's, and that he must have put the gilding on to please the patron of the work," she said.

Ms Dunkerton said the faces of the angels and the saint, which corresponded to those in another Botticelli in the National Gallery, had convinced her it was genuine.

Dr Sally Korman, a lecturer in Italian Renaissance art at the Courtauld Institute of Art, London, who helped with the identification, said she was struck by the quality of the work. Botticelli, born in Florence in 1445, is known for his elongated faces. But during this earlier period, he painted broad features, guided by his teacher, Lippi.