Two art historians said yesterday that a mural they believe to have been painted by Leonardo da Vinci over 400 years ago -- and thought lost forever -- has been discovered under a fresco in Florence, Italy.

John Spencer, a Duke University art historian, and Travers Newton, a conservator at Harvard University's Fogg Art Museum, made the announcement in a press conference at Duke.

Spencer said yesterday that the two have discovered something under the visible fresco -- painted by 16th-century Italian painter Vasari -- that they are "99 percent sure" is Da Vinci's "Battle of Anghiara," depicting a Florentine battle for independence. The discovery was made "using ultra-sound and infrared techniques plus chemical analysis of the core samples of the wall of the Pallazo Vecchio," Spencer said.

Positive identification of waht lies beneath the fresco cannot be made until the Restoration Laboratory in Florence can remove the entire upper fresco. "By Christman they should know what is underneath," said Spencer.

"I think the whole thing is a little premature," said David Brown, curator of early Italian paintings at the National Gallery. "The [restoration] work is being done now by the Italians. I don't know what there is to announce until their work is finished."

Brown did say it is very unusual for the Gabinetto di Restauro (the Restoration Laboratory) to decide to pursue such removal.

Asked why the Restoration Lab gave its permission, Brown said, "That's a good question," and added, "I'm hopeful that it is a Da Vinci."

Spencer said that this search for the lost De Vinci has been going on for about six years, with Newton involved for most of that time. According to David Brown, a variety of art experts in the U.S. have been involved in that quest for the Da Vinci.

Spencer said Newton had looked for the painting originally under the Vasari fresco on the east wall of the Palazzo Vecchio, the Florentine city council room.

Spencer said that Newton had looked on the east wall because Vasari had written that he had painted over the Da Vinci. The action was taken apparently because the Florentine grandduke was uncomfortable with a picture in a government building depicting Florentines winning their battle for an independent republic.

But Spencer and Newton found nothing on the east wall and proceeded to the west wall where they found underneath what they now believe to be the Da Vinci, Spencer said that throughout the ages, the painting has been believed to be on the east wall, due to a misinterpretation of Vasari's writing.

Newton presented his information to Italian officials in late 1976, according to Spencer. In spring of 1978, Spencer became involved in the project, helping Newton find more information about the area where he believed the Da Vinci was located. "He was having credibility problems," said Spencer. Last June, Italian officials decided to go ahead with extensive removal of the top fresco, said Spencer, and they began in September.

Newton has received funding from such sources as the Kress Foundation and philanthropist Armand Hammer, according to Spencer.

"There' no question Da Vinci painted a picture called "Battle of Anghiari," said Brown. "The question is whether Vasari destroyed it or painted over it with his own work. He could have put a ne formulation on the wall or scrpaed off the Da Vinci."

Brown added, "I don't think Vasari-specified it was the east wall. One assumed it was the east wall. It ws supposed to be next to a Michelangelo that was never painted."

Spencer said yesterday that if the mural is found, it will be "as important as the Mona Lisa."