It was lost for so long that it had assumed mythical status for art historians. Some doubted whether it even existed.

But a 500-year-old mystery was apparently solved today after a painting attributed to Leonardo da Vinci was discovered in a Swiss bank vault.

The painting, which depicts Isabella d’Este, a Renaissance noblewoman, was found in a private collection of 400 works kept in a Swiss bank by an Italian family who asked not to be identified.

It appears to be a completed, painted version of a pencil sketch drawn by Leonardo da Vinci in Mantua in the Lombardy region of northern Italy in 1499.

The sketch, the apparent inspiration for the newly found work, hangs in the Louvre Museum in Paris.

For centuries it had been debated whether Leonardo had actually had the time or inclination to develop the sketch into a painted portrait.

After seeing the drawing he produced, the marquesa wrote to the artist, imploring him to produce a full-blown painting.

But shortly afterwards he embarked on one of his largest works, The Battle of Anghiari on the walls of Florence’s town hall, and then, in 1503, started working on the Mona Lisa.

Art historians had long believed he simply ran out of time — or lost interest — in completing the commission for Isabella d’Este.

Now it appears that he did in fact manage to finish the project — perhaps when he encountered the aristocrat, one of the most influential female figures of her day, in Rome in 1514.

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