The self-portrait is on the back of the canvas with Head of a Peasant Woman and is covered by layers of glue and cardboard. Experts believe these materials were applied ahead of an exhibition in the early 20thcentury.
Van Gogh often re-used canvases to save money. However, instead of painting over earlier works, he would turn the canvas around and work on the reverse.
It may be possible to uncover the hidden self-portrait, but the process of removing the glue and cardboard will require delicate conservation work.
Research is ongoing as to how that can be done without harming Head of a Peasant Woman. Until then, the world can enjoy the tantalising discovery through a ghostly and utterly compelling x-ray image.
The newly discovered image shows a bearded sitter in a brimmed hat with a neckerchief loosely tied at the throat. He fixes the viewer with an intense stare, the right side of his face in shadow and his left ear clearly visible.