Fading beauty: Is Mona Lisa losing her smile?

The famous smile of the Mona Lisa maybe in danger of losing its allure. Officials at the Louvre in Paris have ordered tests to check the condition of Leonardo da Vinci's 500-year-old masterpiece after detecting signs of wear and tear.

The warp in the thin poplar wood panel on which it is painted was "greater than that which has been previously observed" in the painting's yearly checks, causing "some worry" at the museum, it was announced yesterday.

The Centre for Research and Restoration of Museums of France will try to determine more exactly what materials the painting is made of and evaluate its vulnerability to temperature change.

The 30-inch by 21-inch painting sits in an air-conditioned glass case behind a waist-high barrier and is seen by about six million visitors each year. It will remain on view throughout the tests, which will begin in 2005.

Over the years, the painting has taken on a brownish cast due to the build-up of dirt and chemical changes to the varnish covering its surface, but the museum has so far resisted pressure to restore the picture to its original colours.

Painted between 1503 and 1506, the Mona Lisa, is also known as La Gioconda, in reference to its probable subject, the Florentine wife of Francesco del Giocondo. It moved to France with da Vinci in the 16th century, where it has remained - except for a short spell when it was stolen in 1911 before being discovered two years later in a Florence hotel.