The art world was stunned earlier this year by the sudden and mysterious postponement of the unveiling of the Salvator Mundi by the Louvre Abu Dhabi. The painting, which had been sold for a record-breaking $450m (£342m), had become the subject of an increasing number of claims by experts that, far from being Leonardo’s forgotten masterpiece, it might not have been his work after all.

Now a French auctioneer has put another Leonardo up for sale, a small drawing of St Sebastian that is estimated to fetch more than €20m (£17.7m). And a leading Oxford art historian who was among those to cast doubt on the Salvator Mundi’s authenticity is questioning whether this picture, too, should be fully attributed to Leonardo.

Dr Matthew Landrus, a Leonardo scholar who believes that most of the Salvator Mundi was painted by an imitator, Bernardino Luini, has identified a “less confident hand” in the drawing of the martyred saint bound to a tree.

He told the Observer: “Though the composition is related to a small group of similar St Sebastian compositions by Leonardo, the darker lines of the figure appear to be by a later hand. The marks are less confident and the upper abdomen muscles are disproportionate in a manner not characteristic of Leonardo’s draughtsmanship, and not necessarily as good as the work of his best associates. This is partially because they are nervously traced over much lighter lines that may have been autograph.”

He said: “What the art market cannot deal with appropriately is the way in which studio assistants and others have contributed to Leonardo’s drawings.”

The Tajan auction house in Paris confirmed last week that it would sell the St Sebastian next June. The drawing, which measures 7.5 x 5 inches, first came to light in 2016, having been in a private French collection since the early 20th century. It was initially declared a national treasure, giving French museums time to acquire it. They did not – and Landrus has suggested that the French government either decided it could not afford it or that the drawing may not be entirely Leonardo’s.

The disputed study of Saint Sebastian which the auctioneers claim is by Leonardo da Vinci. Photograph: Tajan auction house

In announcing the auction, Thaddée Prate, Tajan’s director of Old Masters, recalled that he and fellow expert Patrick de Bayser were “overcome by emotion” in realising that they were “facing a Leonardo study”.

They have liaised with another expert, Dr Carmen C Bambach of the Metropolitan Museum in New York, who has described the attribution as “quite incontestable”. She published St Sebastian drawings – today in Hamburg and Bayonne – in the catalogue for her 2003 exhibition. The drawings are believed to relate to Leonardo’s unexecuted or lost painting of the subject.

Landrus, a research fellow at Wolfson College, Oxford, who has published books on Leonardo, paid tribute to Bambach’s expertise, noting that she was yet to publish a study of this drawing.

Asked whether he sees another hand in Leonardo’s other St Sebastian drawings, he said: “No, just the one.”

This drawing is pasted to another sheet with notes and diagrams about light and shadow, relating to Leonardo’s study of optics. Although Landrus believes that these are certainly by him, he said: “Attaching drawings in this manner seems to have been a much later development.”

Mystery now surrounds the whereabouts of the Salvator Mundi, which portrays Jesus gesturing in blessing with his right hand. It was sold last November by Christie’s New York. Acquired for the Abu Dhabi Department of Culture and Tourism,its planned unveiling in September at the Louvre Abu Dhabi was suddenly cancelled without explanation.

While some experts confirmed the Leonardo attribution in 2011, prominent doubters include Bambach, who believes that “much of the original painting surface” may be by Boltraffio, another Leonardo assistant.

Michael Daley, director of ArtWatch UK, has conducted research into the Salvator Mundi, challenging its attribution. He too has doubts about the St Sebastian drawing: “I think it’s a pastiche. A very beautiful St Sebastian drawing by Leonardo in Hamburg is all very lucid. This one is an absolute dog’s breakfast of a drawing.”

He explained that, while the Hamburg figure has his hands tied behind a tree, this one has one arm raised up, which should have altered the figure’s geometry: “But the two breast muscles of the St Sebastian are in the same position for both versions.” He also pointed to the repeated attempts to draw the figure’s legs: “There are as many legs as a millipede.”

De Bayser dismissed the idea of an additional hand: “There is just one hand, but there are two different inks … like one in the British Museum, the study for the Virgin [and Child] with St Anne, and another in the Louvre … It’s absolutely Leonardo, a fantastic drawing.” He argues that the artist was simply trying out ideas.