GENEVA: A Swiss foundation was set to unveil Thursday what it says is an earlier version of the "Mona Lisa" painted by Leonardo da Vinci, although some experts said the claim was unlikely.

The Zurich-based Mona Lisa Foundation said it would present "the stunning portrait of Lisa del Giocondo," along with results from 35 years of research and scientific tests indicating that "it was indeed executed by Leonardo approximately a decade earlier than its famous sister in the Louvre."

The presentation Thursday afternoon in Geneva would show "historical, comparative and scientific evidence, which demonstrates that there have always been two portraits of the Mona Lisa by Leonardo da Vinci, the 'Earlier Version' and the 'Jaconde'," the foundation said in a statement.

The painting's owners remain anonymous, so it is unclear who would benefit from the painting being attributed to da Vinci.

However, several experts said they suspected the Italian master had not painted an earlier version, the so-called Isleworth Mona Lisa.

"The Isleworth Mona Lisa miss-translates subtle details of the original, including the sitter's veil, her hair, the translucent layer of her dress, the structure of the hands. The landscape is devoid of atmospheric subtlety," Oxford University art historian Martin Kemp said in a statement.

"The head, like all other copies, does not capture the profound elusiveness of the original," he added.

Several other experts also voiced skepticism to AFP about the "earlier version" claim, but they refused to be cited.

Paris's famed Louvre museum meanwhile refused to comment on whether it believed the painting set to be displayed Thursday was in fact an earlier version of the world famous Mona Lisa it has on display.

Kemp meanwhile pointed out that it is known changes were made in the Louvre painting. The Isleworth picture follows the final state of the Louvre painting, he said, insisting: "It does not therefore precede the Louvre painting."

The Mona Lisa Foundation said it would release a 320-page book entitled "Mona Lisa -- Leonardo's Earlier Version", encompassing evidence that da Vinci painted the work, at the same time as the unveiling.

Among the participants in Thursday's media event will be Alessandro Vezzosi, the director of the Leonardo da Vinci Museum in Vinci, Italy, and a world-renowned expert on the artist and the Mona Lisa, the foundation said.