A work of art by Vincent Van Gogh that had been previously unattributed to the artist because it isn't signed has now been shown to be a painting by the 19th century Dutch master. 'Sunset at Montmajour' depicts a winding wilderness of oaks and a stream with fluffy clouds scudding across a pale blue sky.

The painting's authenticity had previously been rejected by Amsterdam's Van Gogh Museum because it had not been signed. The painting spent years in the possession of a private Norwegian collector but he'd kept it in the attic after being told that it wasn't a Van Gogh, according to BBC News. Thanks to new research techniques that took two years to complete, the painting's authenticity has now been confirmed in what the Museum's director Axel Rueger calleda "once-in-a-lifetime experience'' at a recent unveiling ceremony.

Though '...Montmajour' isn't signed, researchers have a pretty good idea of when the painting was created, as Van Gogh's letter to his brother has revealed a date - the 4th July 1888. Writing to Theo, Vincent even described the painting to his brother, saying that he'd painted it "on a stony heath where small twisted oaks grow." The details from the letter had been previously thought to refer to his piece 'The Rocks' although some of the details didn't match. Vincent also confessed that the painting was "well below what I'd wished to do," and gave it to his brother to keep.

Thanks to Van Gogh's letter, the scene depicted in '...Montjamour' has been located as being near Montmajour hill, near Arles in France, where the artist was living at the time. However, the style and materials that were used to create the painting have also served to elucidate the piece's true creator. Researcher Teio Meedendorp explained that the pigments used in the work were colours Van Gogh "habitually had on his palette at this time," including a cobalt blue he reportedly began to use from the summer of 1887 onwards.

A number on the back of the canvas, 180, also matches the painting's position amongst Theo Van Gogh's collection. It was in his possession until it was sold in 1901 and vanished, only emerging in 1970 as part of the estate of Norwegian industrialist Christian Nicolai Mustad upon his death. The family tried to get the painting verified but were dismissed in 1991 after it was decided it wasn't a Van Gogh and it was banished to an attic.

Van Gogh sold one painting in his life and only just began to see acclaim before he died but now his paintings are amongst some of the world's most valuable, selling for tens of millions and attracting visitors wherever they hang.

'Sunset at Montmajour' will be on display at the Dutch museum from 24th September.