A painting long thought to be the work of Dutch master Vincent van Gogh has been proven a fake after a series of tests by art experts in Amsterdam, Australian gallery officials said on Friday.

The painting, titled Head of a Man, has been in the possession of the National Gallery of Victoria in Australia's second largest city of Melbourne since 1940 and was estimated to have been worth A$25 million ($21.4 million) if authentic.

Gallery officials on Friday said while they were disappointed at the finding, experts at the van Gogh museum in Amsterdam found the work was painted during the artist's lifetime, although it had stylistic differences to van Gogh's work.

The reattribution of paintings is part of the daily life in any major gallery with a large and complex collection, National Gallery of Victoria director Gerard Vaughan said on Friday.

Doubts were raised about the painting's authenticity a year ago when critics viewed it at an exhibition in Edinburgh, Scotland, prompting its owners to send the work to the van Gogh museum in Amsterdam for further examination. The painting, portrait of an unknown man, was first brought to Australia by the late newspaper publisher Keith Murdoch in 1939 as part of a traveling exhibition.

Vaughan said the painting was purchased as a van Gogh, and had been accepted as a van Gogh for more than a decade before it was purchased.