Image caption Vincent van Gogh's self-portrait (l) with the painting of his brother Theo (r)

A Vincent van Gogh painting, which had always been believed to be a self-portrait, is actually a depiction of his brother, experts have concluded.

The artwork, which is on display at Amsterdam's Van Gogh Museum, is thought to be the only known painting of Theo.

"People have often thought it was funny that there were no portraits of Theo, given that they were so close," said museum spokeswoman Linda Snoek.

It is thought the 1887 work was created when the pair lived together in Paris.

The museum's head researcher Louis van Tilborgh made the discovery after comparing it to another Van Gogh self-portrait.

"They are two small, detailed portraits that when you see them you think they belong together," Mr Van Tilborgh said.

While the brothers appear to be physically similar, he discovered there were a number of differences between Vincent and his brother's features, including different ear shapes and the colour of their beards.

Theo also has shaven cheeks, which match up with photographs taken of him at the time, while he has rounder ears.

The portrait of Vincent which famously depicts him after mutilating one of his own ears was created later.

The painting has been in storage, but is being displayed at the Dutch museum as part of an exhibition on new findings about time the painter spent in Antwerp and Paris during 1885-1888.

The artist shot himself in 1890 and died shortly afterwards.