One of J.M.W. Turner’s most self-portraits, which appears on the £20 note, may not be by the artist at all.

Art historian James Hamilton, who has written widely on Turner, curated exhibitions on the artist, and authored the 1997 biography Turner—A Life, told the Guardian this week that he believes the painting has been misattributed to Turner. While the painting, dated to ca. 1799, does depict the English Romantic painter, he said, he believes it to have been painted by John Opie.

Hamilton said he has been researching the portrait for years, after using it on the cover his biography, and now believes the painting was misattributed when it was included in the Turner Bequest, in which the aritst gifted 300 oil paintings and 30,000 sketches and watercolors to the state after his death in 1851.

“Turner’s relations challenged the will and, after a long, tortuous court case, the judge said the family can have the money and the nation gets the pictures—not only the ones that he wanted the nation to have, but everything by his hand in his studio,” Hamilton said. “There were many pictures hanging in disarray in Turner’s house in Queen Anne Street.”

Given its ca. 1799 dating, the work would have been painted when Turner was around 24. Hamilton said that the style of the work is that of Opie, who was a master portraitist and 14 years Turner’s senior. Opie typically depicted his subjects in “light emerging dramatically from dark,” as in the portrait in question.

Hamilton’s research was first published this week in Turner Society News, and he has called on the Tate, which owns the work, to reattribute the painting to Opie. However, two Turner experts expressed skepticism about Hamilton’s findings. The Tate told the Guardian that it plans to explore Hamilton’s research soon.