The ensuing battle to prove its provenance has pitted Chagall’s granddaughter against one of Britain’s leading art historians, and called into question the murky processes through which the art world judges whether paintings are fakes or the real thing.

Martin Kemp, emeritus professor of the history of art at Oxford University, was convinced that the St Matthew painting was genuine.

With his arched eyebrows, slender face and long, straight nose, the gospel writer bore a striking resemblance to Chagall himself. And as in so many of Chagall’s paintings, the angelic muse at his shoulder had to be his wife, Bella.

The back story was also plausible. The painting had belonged to Mary Gillies Pollitzer, a wealthy Jewish woman who had a summer retreat in Saint-Paul de Vence, a medieval town near Nice where Chagall settled in 1950. Her brother-in-law had exhibited his paintings alongside Chagall at the Redfern gallery in London. Chagall is also known to have visited Kent in the 1960s to create stained-glass windows for All Saints’ church in Tudeley, including one of St Matthew.

The Comité Marc Chagall was not impressed. In a letter seen by The Times, Meret Meyer Graber, its vice-president and one of Chagall’s granddaughters, dismissed the claim, saying that the style, composition and provenance of the painting were “far removed” from the artist’s oeuvre.

The dispute came to light at the Cheltenham science festival this week. Art experts are increasingly concerned about the extent to which authentications potentially worth millions have come to rely on the sole authority of artists’ estates.

The Warhol Foundation stopped verifying work after becoming caught up in a series of expensive lawsuits in which its opinions were cited as pivotal evidence. Researchers say that guardians of artists’ legacies often dismiss artworks out of hand, sometimes because they have a vested interest in maintaining the value of “official” paintings.

Chagall is one of the most heavily forged painters of the past 100 years: at least two people have been jailed for selling copies of his work. In 2014 the Comité Marc Chagall ensured that an apparent fake owned by a Leeds businessman was burnt against his wishes.

Two years later Professor Kemp and a colleague tried to publish an academic article setting out the case for the St Matthew painting’s authenticity. Professor Kemp alleges that Ms Meyer Graber and the committee tried to “censor” the paper by unsuccessfully lobbying the editor of Kunstchronik, a German journal, not to publish it.

In 2008, Professor Kemp wrote to Ms Meyer Graber accusing her of failing to live up to the standards expected of art authorities. “Matters of attribution are matters of opinion, and no one person or organisation, whatever its family connections with the artist, is assigned or should be assigned the sole right to assert definitely what is authentic and what is not,” he wrote.

The Times was unable independently to verify the claim that Ms Meyer Graber sought to prevent the article’s publication. Calls to the committee’s address in Paris and attempts to contact its members went unanswered yesterday.