The Spaniard’s paintings sell for millions, but a British expert now claims that many pieces attributed to him were by his assistants

The authenticity of dozens of Goya paintings is being questioned by a leading British art historian who claims that they are not in fact works by the Spanish master but instead created by some of the great painter’s many studio assistants.

Juliet Wilson-Bareau, a Goya scholar, told the Observer that museums must re-examine their Goya holdings because there are so many “problematic” pictures. She regularly sees auction houses and dealers selling works under Goya’s name when she is convinced that they are by lesser hands.

“It’s a minefield,” she said. “Pictures that have become doubtful are still being presented as ‘by Goya’, knowingly or unknowingly. Everybody wants a picture to be by Goya.”

Francisco de Goya, who lived from 1746 to 1828, is revered for masterpieces including his savage satires of man’s inhumanity to man in the Disasters of War series.

Facebook Twitter Pinterest Goya’s Bulls in a Town, 1808-1812. Photograph: Prisma Archivo/Alamy

Wilson-Bareau, who has lectured on Goya at Oxford University as a Slade professor, has curated major Goya exhibitions at the Prado, Madrid, and the Royal Academy of Arts, London. She estimates that “dozens” of Goyas will have to be reattributed and that it will take years of research: “We’ve been aware of the problems for many years, but everybody’s got to do their research now. Almost every museum that’s got a Goya has also got a problematic Goya. They’re very widespread. Some are quite clearly later fakes, but many were by his studio assistants.”

Part of the problem had been an 1812 inventory of Goya’s studio and other possessions, she said. “We believed that everything in that inventory must be by Goya. Over time, I have come to the conclusion that some of them could not be by his hand.

“The answer is that Goya, like any great artist, had an active studio with assistants, and that many of the paintings would therefore be by other people, but they would all go out of the studio as by Goya.”

Facebook Twitter Pinterest Francisco de Goya, self portrait, 1783. Photograph: © Musée des Beaux-Arts d'Agen

Goya was known to have had assistants, she said, but up until recently the role these people played was ambiguous. “Now it’s clear, partly through documentary evidence that proves people were buying Goya and non-Goya works under the same label, that Goya was, in effect, heading a school, as noted by a contemporary.”

She added: “The Metropolitan Museum, New York, has already downgraded several of its paintings as not by Goya, but without being able to confirm their date or authorship. Convinced by this idea that Goya did have a studio – which could account for problematic pictures – the Met will now re-examine [them].”

They include a large painting of Majas on a Balcony, a replica of Goya’s original in a private European collection. Bareau-Wilson said: “Another painting of the same subject was included in the Louvre’s 2013 Goya ‘study days’. I believe it is by Goya’s close assistant Asensio Julià, who may therefore have painted the more Goyesque version in the Met.”

Julià features in a major exhibition on Goya’s studio that Bareau-Wilson has curated for the Museum of Fine Arts in Agen, in south-west France. Goya: Avant-Garde Genius. The Master and his School features nearly 90 works loaned by museums and private collections worldwide.

Facebook Twitter Pinterest Majas on a Balcony by Asensio Julià, Goya’s assistant. Photograph: © Collection particulière, Paris

Goya masterpieces hang alongside pictures which Bareau-Wilson believes are by his assistants, although they came from his studio: “That’s why people are able to cling to the fact that they are Goya, even when some of us can see that they aren’t. This is new research.”

Many of the dubious works were included in the 1970 definitive catalogue raisonné, on which she worked as a young art historian with Pierre Gassier: “A huge number I’d only seen from photographs.”

She spoke of her “trepidation” about challenging attributions as Goya’s pictures change hands for millions of pounds: “If a picture turns out to be by an assistant, of course the value collapses. “Artists in the shadow of a great master [can] illustrate his imagery, his style, his handling of paint, but the work is not infused with the unique individuality and strength of the original creator.

“The more works that are identified as being not by Goya, the higher the quality of his own work becomes. That’s what inspired me.”

She will publish her research in a forthcoming catalogue for the Agen exhibition, which is running until 10 February.