Forgotten Artemisia Gentileschi painting found in Hampton Court storeroom | Painting

“A woman’s name raises doubts until her work is seen,” wrote the artist Artemisia Gentileschi to a collector of her paintings in 1649, going on to assure him that her canvases “will speak for themselves”. It took three-and-a-half centuries for the name of Gentileschi to triumphantly step out from the shadows of art history, but it has taken even longer for one of her forgotten paintings to re-emerge from the dark. A remarkable find made in a royal storeroom at Hampton Court, followed by hours of careful conservation effort, has led to the unearthing of Susanna and the Elders, a genuine lost Gentileschi.

“It really is super-exciting,” Anna Reynolds, the deputy surveyor of the king’s pictures, told the Observer. “You just could not see the quality of the painting beneath the grime until now, but absolutely it is true and this find has come about as a result of Artemisia’s recently restored reputation. It had been misattributed and left in storage for many years and no one had taken a closer look.”

The rediscovered painting, put up on show at Windsor Castle in the last few days, is regarded as a key addition to Gentileschi’s body of work, shedding light on her time in London in the late 1630s, when she briefly worked alongside her father, Orazio, at the English court. More remarkably still, its subject goes straight to the heart of the artist’s own concerns. Infamously, the teenage Gentileschi had been raped in Italy by another artist, Agostino Tassi, in her father’s workshop and then interrogated and tortured at his trial. Archived trial transcripts show Tassi was eventually convicted.

The painting depicts the biblical story of Susanna, who was said to have rejected the advances of two men in her garden and then faced death after a false accusation of infidelity was made. Gentileschi gives unusual emphasis to Susanna’s efforts to evade the men in this treatment of the story, one of several by the artist, and Reynolds believes “she seems to have felt a close affinity with Susanna”.

The discovery, made by Royal Collection Trust curators, in particular the art historian and former staffer Dr Niko Munz, came during attempts to trace all the paintings sold off across Europe after Charles I’s execution, which included seven recorded Gentileschi paintings. Only a well-known self-portrait was thought to have survived.

“We were going through the inventories and Dr Niko noted one that was like something kept in the Hampton Court store for more than 100 years,” said Reynolds. And Munz was right. Shrouded in dirt, deep beneath the heavy varnish of previous restorations, lay a striking image now thought to have been commissioned by Charles I’s wife, Henrietta Maria of France.

“It was incredibly brown and discoloured, as often happens with organic varnish. Its size had also been increased with strips of sewn-on canvas, which could have been the reason for all the over-painting,” recalled Reynolds. “It was only once we started to see the work underneath that a few special elements stood out.”

Among the clues was the use of the paint colour lead-tin-antimony yellow, one of the artist’s favourites.

Experts were called in to confirm the finding and then, more conclusively, fabric removed from the back of the canvas revealed the letters CR for Carolus Rex, representing Charles I.

Gentileschi had been invited to London at a time when her fame was at its height. In later years, however, interest in baroque 18th-century paintings fell away and her work was stored and misattributed to another, lesser Italian artist, before then being wrongly ascribed to the French School.