Not just Mona Lisa's smile? Da Vinci's La Bella Principessa also has 'uncatchable' expression

It is the most-visited, the most written about and the most parodied work of art in the world but the ‘uncatchable smile’ that makes Leonard da Vinci’s Mona Lisa so special is not unique according to experts.

Another painting, La Bella Principessa, which predates da Vinci’s most famous work, also has a smile which seems to change according to the viewpoint of the onlooker.

Academics suggest that the technique which would later give his most famous subject her mysterious allure was first executed on the lesser-known painting by the Renaissance master.

When viewed directly the slant of the subject’s mouth appears to turn downwards, but as the eye moves elsewhere to examine other features, the edges of her mouth appear to take an upward turn. Her sideways smile can only be seen indirectly, much like Mona Lisa's.

The painting is thought to depict 13-year-old Bianca Sforza, the daughter of Ludovico Sforza ("Il Moro") duke of Milan, before her marriage to a commander of the Milanese forces. It is made more poignant by the fact that the girl died within months of her marriage, having suffered a suspected ectopic pregnancy.

"The results from the experiments support the hypothesis that that there is a gaze-dependent illusory effect in the portrait of La Bella Principessa,” said Alessandro Soranzo, from Sheffield Hallam's department of psychology, who co-authored the study.

"Although it remains a question whether the illusion was intended, given Leonardo’s mastery of the technique and its subsequent use in the Mona Lisa, it is quite conceivable that the ambiguity of the effect was intentional, based on explicit artistic skill and used in line with Leonardo’s maxim that portraits should reflect some ‘inner turmoil of the mind’."

Co-author Michael Pickard, from the University of Sunderland, said: "With his knowledge of the turbulence surrounding the Court of Milan at that time, Leonardo would have been aware of inner tensions between the fresh innocence of a young girl on the threshold of womanhood and her impending marriage and courtly destiny.”

"It is also not difficult to believe that Leonardo would have seen below the surface and wanted to capture the subtle essence of the girl, using a technique he would so famously master in the Mona Lisa."

The Mona Lisa, which is around 500 years old, is the star attraction at Musée du Louvre in Paris. The small portrait of an obscure Florentine gentlewoman, Lisa Gherdardini, is thought to attract 90 percent of the museum's intake - despite it housing 6,000 other paintings.

Archaeologists hope that one of two skeletons unearthed in a Tuscan convent in October will be shown to be that of the model who became Mona Lisa. In their hunt for the remains of the most famous portrait-sitter in history, experts have been digging in the former convent of St Ursula in Florence since April.

In September a Swiss foundation unveiled what it said was an earlier version of da Vinci's masterpiece, claiming it was completed over a decade before the famous Renaissance painting.

The work, known as the Isleworth Mona Lisa, was exhibited publicly in Geneva by the Mona Lisa Foundation, which claims that 35 years of research, numerous tests and mathematical comparisons have led experts to conclude that it predated the Louvre painting by 11 to 12 years.