The painting attributed to Leonardo da Vinci claims to be of the same lady as in the Mona Lisa, but in her 20s. Photograph: Denis Balibouse/Reuters

Portrait claimed to be of younger version of painter's muse to be unveiled in Geneva by art foundation

For generations the enigmatic smile of the Mona Lisa has captured the imaginations of thousands of visitors to the Louvre. But now the mythology surrounding the world's most famous painting is being questioned by the unveilling of a supposed second version of the portrait.

The painting, purporting to show a younger portrayal of Leonardo da Vinci's muse, will be presented in Geneva on Thursday by the Swiss-based Mona Lisa Foundation.

It claims that three decades of research suggests that the painting is an earlier version by the Italian Renaissance artist.

"We have investigated this painting from every relevant angle and the accumulated information all points to it being an earlier version of the Giaconda in the Louvre," foundation member and art historian Stanley Feldman, told Reuters.

The foundation contends that the portrait shows a woman who appears to be in her early 20s, a decade younger than the version in the Louvre.

But other art experts are highly sceptical and contend the work is more likely to be a later copy.

Oxford University professor Martin Kemp said the Geneva portrait was probably a copy of the Paris version by an unknown painter who simply chose to make the subject younger, Reuters reported.

"So much is wrong," said Kemp, a world-recognised authority on Leonardo, noting that the foundation's portrait is painted on canvas and not on wood, the artist's preferred medium.

The "younger version" – which the Foundation says was probably painted around 1505 – is not new to the art world but has been locked away in a Swiss vault for many years.

It was discovered in 1913 by collector Hugh Blaker in a manor house in the west of England where it had hung for a century unnoticed.

Blaker took it to his home in a London suburb, where it was dubbed "the Isleworth Mona Lisa". On his death in 1936, it was bought by American collector Henry Pulitzer. It is currently owned by an international consortium.

The Louvre Mona Lisa, which was in Leonardo's possession when he died, is known as La Giaconda or La Joconde after Lisa Gherardini, wife of the early 16th century Italian nobleman Francesco del Giacondo, who commissioned a portrait of her.