(Reuters) - Leonardo da Vinci is sick and no one really knows if he ever will be able to receive visitors again.

Art conservation and restoration experts recently concluded weeks of tests on the famous self portrait of one of history's greatest geniuses, sketched in the early 1500s when he was in his 60s.

And the diagnosis is decidedly grim.

The non-invasive studies confirmed art experts' worst fears: the drawing is seriously damaged and deteriorating and any restoration would be delicate and risky to say the least.

"I think we need to think very hard before we do anything to this very familiar face," said Jane Roberts, Royal Librarian and Curator of the Print Room at Windsor Castle.

"But we can tell quite a lot more about it by continuing to ask questions," she told a news conference in Rome.

The small drawing of the Renaissance master, which measures 33.5 by 21.6 centimeters (13.2 by 8.5 inches), shows Leonardo with pensive, baggy eyes, bushy eyebrows and a flowing beard.

The self portrait, done with red chalk on paper, is suffering from what the art restoration world calls "foxing," a generic term for blotches, spots and stains -- marks that should not be there.

Foxing can be caused by oxidation of the pigmentation Leonardo used as well as fungi on the paper, made of hemp, flax and wool, or rust from the iron in the pigments.

Leonardo's forehead, aquiline nose and puffy cheeks look like he has a bad case of the measles.

"Because this is a masterpiece, prudence has prevailed," said Maria Cristina Misiti, head of Italy's Central Institute for Restoration and Conservation of Archival and Book Patrimony.

"It scary to deal with a work of art of this magnitude and uniqueness," she said.

The decision on whether to restore the drawing would be a difficult one to make, and would be taken by the Royal Library of Turin, the restoration institute, and scientists, she said.

The drawing was acquired by King Carlo Alberto of Savoy in 1839 and was well preserved in the Royal Library for nearly 100 years. But in 1929 it was framed and put on a wall, exposing it to sunlight.

"We will continue to study it, to diagnose it. Everyone agrees on that," Misiti said.

The drawing, which is kept in a vault in Turin, was shown in an extremely rare exhibit for two months last year in connection with celebrations marking the 150th anniversary of the unification of Italy.

But art experts say that any other exhibition would be for short periods of time and with a limited number of visitors, because, like any sick old man, Leonardo will need what art experts call "long periods of rest".

(Reporting By Philip Pullella; editing by Patricia Reaney)