National Gallery’s 1450 portrait by Rogier van der Weyden was created in the 1960s by Eric Hebborn, says art historian

To the National Gallery, the man depicted in the masterpiece that hangs in its gallery of 15th-century treasures is a holy man, possibly a saint, reading a legal text. And the portrait is believed – at least by the gallery’s experts – to have been created in the workshop of the Netherlandish painter Rogier van der Weyden.

But to one leading art historian, it is nothing of the sort. Instead, it is a 20th-century fake, of an unknown man sporting a Beatles-style haircut and reading a paper containing nothing more than nonsense.

And, claims Christopher Wright, an old-masters scholar, its likely creator is Eric Hebborn, the greatest forger of modern times. Wright is challenging the attribution of A Man Reading, possibly Saint Ivo, which the gallery label dates to “about 1450”.

He told the Observer that the picture “screams” the 1960s, and that Hebborn had repeatedly claimed authorship – before denying it.

The painting is thought to represent the patron saint of lawyers and advocate of the poor. The figure appears to be holding what the gallery describes as “a legal document, which would be appropriate for Saint Ivo”.

Wright ridicules the haircut of the figure who is reading a text that is “gobbledegook” – “an impossibility for a long inscription in that period when artists only wrote inscriptions to be read”.

Teasing the art world was part of Hebborn’s trade, both before and after he was unmasked in 1978 as a brilliant forger. He fooled the world’s foremost galleries and auction houses with forgeries in the style of Rubens and Van Dyck, among others.

“Only a handful have been exposed,” Wright said. “There are still Hebborns floating around the art market and in museums.”

For Hebborn, “the greater the expert, the greater the satisfaction of deceiving them”.

The art world was too humiliated to press charges. He died mysteriously in 1996, found lying on a street in Rome with a broken skull.

Wright first briefly challenged the authenticity of the Saint Ivo 35 years ago in his book, The Art of the Forger, without suggesting Hebborn’s name. But it prompted the then National Gallery director to cancel Wright’s lunchtime lectures and order the bookshop to remove all copies – although staff held copies under the counter for visitors requesting one.

Wright said that subsequent dendrochronological dating of the picture’s oak panel found that it dates from about half a century after the artist’s death, “which is why it’s attributed to his workshop”.

Just as Hebborn used old paper for drawings, he could use wood from old furniture, “which is not difficult”, he added.

He pointed to other stylistic problems, such as the figure’s hood, which defies the laws of gravity, and a modern-looking facial scar: “The accumulation of small pieces of evidence build up to the thing being wrong.”

He said: “If I was the gallery director, I would simply say that some have thought it to be more modern.” He understands the gallery’s reluctance: “To come out and say ‘Well, actually, we bought a 20th-century fake’ is not really a thing that they would do.”

The painting, which measures 45cm by 35cm, was acquired in 1971. Wright believes that it had been planted in a peer’s outhouse for a dealer, David Carritt, to find in 1968.

A footnote in the National Gallery’s 1997 technical bulletin refers to “baseless claims that it is a modern forgery” in Wright’s 1984 book and a 1991 Hebborn interview. The text notes that it has been called a “20th-century forgery in spite of the fact that it belonged to a collector who died in 1806”.

But Wright argues that there is no evidence that this is the same painting. He believes that an 1850s drawing by Sir George Scharf of a similar composition relates to a lost painting as it corrects mistakes in the National Gallery painting: “In the drawing, the saint is directly looking at the paper, which he’s not in the painting. The perspective of the writing is also wrong in the painting. Hebborn made mistakes as he wasn’t steeped in this period.”

His doubts about the Saint Ivo have resurfaced because he is now finishing his memoirs. Remembering that Hebborn had also made the authorship claim to a documentary-maker off camera, he found clues while re-reading Hebborn’s books, his 1991 autobiography, Drawn to Trouble, and The Art Forger’s Handbook, published posthumously.

“There is quite a lot about forging early paintings in the book,” he said. “Hebborn says things that imply he did it. In one passage, he writes that he is painting an early Netherlandish picture for a dealer. So is that a hoax?”

The National Gallery said: “The claims that [it] … is an Eric Hebborn forgery are baseless. The picture is painted on an oak panel which has been dated by dendrochronology... to the mid-15th century... The pigments, paint medium and layer structure are all fully-compatible with a likely date of about 1450. The painting is first recorded in 1801 and has a full and unbroken provenance until the present day.”