National Gallery (search) experts using infrared techniques have discovered a Leonardo da Vinci (search) sketch hidden underneath a painting by the Italian master, conservationists said Friday.

The sketch — the first unknown Leonardo image to be found in decades — is beneath the delicate brushstrokes of the artist's "Virgin on the Rocks (search)," a powerful scene of Christ's mother in a dusky cavern, which hangs in the London museum.

The concealed image shows a woman with one hand clutched to her breast, the other outstretched, kneeling before what experts said was planned to be an infant Jesus. Leonardo apparently was planning a picture of the adoration of the Christ child, a scene popular with Renaissance artists, but changed his mind.

"It came as a complete surprise to finding the sketch," said Rachel Billinge, research associate in the conservation department of the National Gallery. "We had no idea until we studied the painting that there was anything under there."

The National Gallery's "Virgin on the Rocks" is a copy of the painting of the same name that now hangs in the Louvre in Paris. National Gallery experts were using infrared techniques to find out how the copy had been made when they found the sketch.

The Roman Catholic church had commissioned Leonardo to paint "The Virgin on the Rocks" for a Milan chapel altarpiece in 1483.

"When (Leonardo) completed the first painting, he was so pleased with it that he asked for more money, and when this was refused, he sold it privately," Billinge said.

The artist later agreed to paint another picture — and probably started with the newly found sketch — but was persuaded to make a copy of the original "Virgin On The Rocks," she said.

The copy was placed in the chapel in 1508.

Critics argue over the meaning of the scene in "The Virgin On The Rocks." Some claim it depicts the Immaculate Conception, while others say it shows the first time Jesus met John the Baptist.

The last time any new Leonardo works were discovered was the 1930s, when the portrait "Ginevra de'Benci" and "The Madonna of the Carnation" were attributed to him.