An early portrait by Lucian Freud, which the artist denied was his for years, is to be exhibited for the first time after experts proved it was painted by him.

Man in a Black Scarf was created in 1939 by the British artist when he was still a student at the East Anglian School of Painting and Drawing in Hadleigh, Suffolk. The sitter is thought to be John Jameson, a friend of Freud’s and scion of the whiskey family.

The work became more widely known after it appeared on the BBC’s Fake or Fortune show in 2016, with the art historian Philip Mould concluding it was very likely a Freud.

But the picture was complicated by the fact Freud had repeatedly denied the work was his before he died in 2011. In 1985, Christie’s identified it as a painting by the artist, but reversed its decision when Freud said he had not painted it.

The denial appeared to stem from Freud’s personal feud with the original owners of the work, Denis Wirth-Miller and Richard Chopping, with whom he attended the Suffolk school as a teenager.

“He was the golden boy, he was a star even then and there was jealousy,” says the designer and author Jon Lys Turner, who inherited the work and claims Wirth-Miller kept a list from their school days titled “13 Reasons to Hate Lucian”.

Wirth-Miller told Turner he could have the painting on the conditions that he’d authenticate and sell it “in order to infuriate Lucian”. Turner then attempted to have the work signed off as a Freud over the course of 19 years – without success, as experts were not willing to publicly contradict the artist.

Two years after Fake or Fortune aired, a new piece of evidence emerged which supported Turner’s case. Students at the Suffolk art school had noted what they were working on at the end of each day, and records held in the Tate Britain archive showed that Freud had been painting John Jameson in 1939.

Man in a Black Scarf will now be shown publicly for the first time in the Benton End: A Paradise of Pollen and Paint exhibition at the Garden Museum in London. The show takes its name from the Suffolk farmhouse that hosted Cedric Morris and Arthur Lett-Haines’s East Anglian School of Painting and Drawing.

Freud also painted Morris a year after the Black Scarf painting. This work is held in the National Museum of Wales and bears stylistic similarities to Man in a Black Scarf.

Turner says the exhibition will establish the overlooked connection between Morris and his student Freud. “[The portrait] has a confrontational gaze and these large eyes and the great thick paint sort of daubed and quite roughly handled. It’s an incredibly astute way of capturing that person,” he said. “He was picking this up from Morris.”

Turner has not had Man in a Black Scarf valued, but in 2016 it was speculated that it was worth more than £300,000. However, Freud works can fetch much larger sums: in 2015, his Benefits Supervisor Resting sold for $56m (£42m), with his auction record at $86m. Next month another Freud work, Sleeping by the Lion Carpet, is being auctioned by Sotheby’s in London with an estimate of £25,000-£35,000.