A Vincent van Gogh’s painting which was thought to be a self-portrait is actually a depiction of his brother, experts have concluded.

The 1887 artwork of a man wearing a light-coloured hat and a dark blue jacket is Theo van Gogh, Vincent's junior by five years.

It’s on display at Amsterdam's Van Gogh Museum, which houses the largest collection of the 19th century master’s paintings and letters, and it is thought to be the only known painting of Theo, crafted when the pair lived together in Paris.

"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.

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.

Theo was Van Gogh’s patron, confidant and companion. In his many letters to him, Van Gogh reveals himself as artist and man, laying bare his deepest feelings and everyday concerns and his views of the world of art.

Theo died six months after his older brother shot himself in a

wheatfield at the age of 37 in Auvers, France, in July 1890.

While the brothers appear to be physically similar, Van Tilborgh 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.