Portrait found among private Swiss collection has a Mona Lisa smile and same paint pigment and primer as those used by artist

Researchers in Italy claim to have unearthed the portrait of a noblewoman by Leonardo da Vinci which has been lost for 500 years and features the same enigmatic smile as his Mona Lisa.

The portrait of Isabella d'Este, which carbon dating suggests was painted around the start of the 16th century, has been found in a vault in a private collection in Switzerland, and has been verified by a leading authority on the renaissance polymath.

"There are no doubts that the portrait is Leonardo's work," said Carlo Pedretti, an emeritus professor of art history at the University of California.

If acknowledged as genuine – and if experts concur that it was painted before the Mona Lisa – the portrait could shake up academic studies of one of the world's most famous paintings.

The 61cm x 46.5cm portrait, which uses the same pigment and primer that Leonardo used, is thought to be the completed version of a sketch he made of D'Este, which, like the Mona Lisa, hangs in the Louvre in Paris.

The unnamed family which owns the portrait, and asked for it to be analysed, has kept a collection of about 400 paintings in Turgi since the start of the 20th century, according to the Corriere della Sera newspaper.

A preliminary sketch of Isabella D'Este. Photograph: Corriere della Sera

In a letter to the owners, Pedretti said he was convinced Leonardo had painted the face, while two assistants were responsible for a palm leaf the subject is holding.

Leonardo first met D'Este in 1499 when he took up residence at her court in Mantua, where she was marquesa. A patron of the arts and a leading figure in the Italian renaissance, whose dress sense influenced women in Italy and France, she sat for the artist and later implored him in letters to turn his sketch into a painting.

Leonardo promised he would complete the commission, at one point suggesting he could work from the sketch without her sitting again. In 1514 it is likely the two met again at the Vatican, but historians have argued that the painting was either never completed or was lost forever.

However, one historical clue suggests Leonardo did complete the work. In 1517, while in France, he showed a series of paintings to Cardinal Luigi d'Aragona, prompting the cleric's assistant to write: "There was a painting in oil depicting a certain Lombardy lady."

Pedretti said that after studying the portrait for more than three years he was now going public with his findings, although he needed a few more months to be absolutely sure which parts of the painting were the work of Leonardo's assistants.