A picture believed to be one of only two surviving photographs of artist Vincent Van Gogh is in fact a picture of his brother Theo, the Van Gogh museum in Amsterdam has announced.

The black and white image had long been thought to show Vincent at the age of 13, and has featured in biographies of the notoriously camera-shy artist behind "Sunflowers" and a series of self-portraits.

But after doubts were raised about the identity of boy in the picture, expert research showed it was not Vincent but his younger sibling Theo, then aged 15.

"When I heard that this is most likely a picture of my great-grandfather Theo, and not of Vincent, I was surprised, but I am glad that the mystery has been solved," Willem van Gogh, Theo's great-grandson and an advisor to the board of the Van Gogh Museum, said in a statement.

Vincent Van Gogh killed himself at the age of 37 in 1890 after an emotionally turbulent life, detailed in years of correspondence with Theo, an art dealer who supported the artist emotionally and financially.

While Vincent Van Gogh is famed for his vividly-coloured, emotionally turbulent self-portraits in oils, he showed a surprising reluctance to be captured on camera.

The only other known photograph of him shows him at the age of 19.

The picture wrongly believed to show Vincent as a 13-year-old, taken by Brussels photographer Balduin Schwarz, first emerged at an exhibition in 1957 organised by a Belgian researcher who identified it in a catalogue as 'Portrait of Vincent Van Gogh (circa 1886)'.

But after a Dutch TV programme raised questions about it, and another Van Gogh expert expressed doubts about when the photo was taken, the Van Gogh Museum launched an investigation.

Confusing the issue was the fact that the brothers looked alike, with reddish-blond hair - but Theo had more delicate features and a "very light eye colour" which looked like it could be seen in the photo, the museum said.

The museum then commissioned a forensic study by a professor at Amsterdam University.

The study showed with a "high degree of probability" that the photo was of Theo.

"With this discovery we are an illusion poorer and a portrait of Theo richer," said Axel Rutger, director of the Van Gogh Museum.

"In fact, we are now back in the situation as it always was before the wrong identification in 1957 - with only one photo portrait of a young, 19-year-old Vincent van Gogh."