It is the world’s most famous smile, in the world’s most famous painting – but it may not be unique.

The debate over the authenticity of the “Isleworth Mona Lisa” looks set to continue, following carbon-dating of the canvas. The copy of Leonardo’s masterpiece was greeted with some scepticism when it was unveiled in Geneva last autumn, with most experts failing to support the claim that the work is by Leonardo. The Swiss Federal Institute of Technology in Zurich has now dated a small sample taken from the canvas to between 1410 and 55, with a 95% probability it lies within this range. The Mona Lisa Foundation, which is promoting research on the Isleworth copy, believes that Leonardo painted the picture shortly before he began the Louvre version in 1503.

The Isleworth Mona Lisa got its name a century ago when it was bought by Hugh Blaker, a critic and restorer, who lived in the west London suburb. The foundation, which is convinced the portrait is by the master, argues that the early date for the canvas is strong evidence against the painting being a later copy. However, it seems curious that Leonardo would have used canvas that was 45 to 90 years old, since the material was relatively cheap. The paints would have been much more expensive and the artist would have taken considerable time on a first version of the composition, making it surprising that he skimped on the support.

The foundation is planning to exhibit the Isleworth Mona Lisa in Singapore next year. The painting is then scheduled to go on to Hong Kong, Shanghai and Beijing, before going to America and Europe.