Mona Lisa has less to smile about

PARIS, April 26 (UPI) -- Mona Lisa's smile may turn into a scowl if Leonardo da Vinci's famous portrait isn't restored, experts at the Louvre museum warned Monday.

"The state of conservation of Leonardo da Vinci's masterpiece is cause for concern," the Louvre said in a statement.

Leonardo painted the 1506 portrait, also known as "La Gioconda," on a thin panel of poplar wood which "is more warped than it was previously," the Louvre said. Experts said the warp was affecting the visual image.

The Louvre has ordered a detailed series of tests, "to understand better the materials used and to determine the actual condition of the painting."

Probably the most viewed painting in the world, the Mona Lisa is sensitive to changes of temperature. It is exhibited behind a special non-glare, protective glass panel. From 2005 the work will be located in its own room as part of a major refurbishment of the Louvre galleries.