Was there a family conspiracy to cover up the truth about Goya's finest work?

They are 14 of Francisco de Goya's finest and best-known masterworks, sombre scenes of violence and horror that brood over the heads of millions of tourists who flock to Madrid's Prado Museum every year and head for the "Black Paintings" as their first port of call.

But are they genuine? Works such asSaturno and Duelo de garrotazos, jewels in the Prado's world-class collection, are not the work of the Spanish master after all, a historian in Madrid claimed yesterday.

The Black Paintings are constantly reproduced as timeless expressions of human alienation and the chaos of war. Saturn Devouring One of His Sons was recently echoed in a polemical cartoon in this newspaper.

But these masterpieces of Goya's mature splendour, seen as reflecting the artist's disillusion with the follies of the world as age and illness subdued him, may instead have been painted by his son Javier, and hyped up in a speculative business deal by the artist's grandson Mariano, the historian argues.

Juan Jose Junquera, an art historian at Madrid's Complutense University, says his investigation of French and Spanish archives throws into doubt Goya's authorship of the oils originally painted on the walls of the artist's house, La Quinta del Sordo (Deaf Man's Estate), near Madrid.

Mr Junquera said yesterday: "It's not certain who painted them. They might have been done by his son Javier, who was an accomplished amateur artist. Even his father said he had talent. But I am sure they are not by Goya."

His article in May's edition of the Spanish magazine Descubrir el Arte is based on his biography of Goya, to be published shortly by the British company Scala.

Javier Goya, the only one of the artist's 20 sons to reach adulthood, "was a self-important nouveau riche who knew his father's work perfectly," Mr Junquera says. "The most surprising thing is the lack of documentary information about these works. Goya made no reference to these murals during his life, and none of his friends ever mentioned them. That's very strange. The first we hear of them is in 1868, 40 years after Goya died, and after the death of Javier in 1854. I've got proof. It's for others to decide."

The historian believes the Prado should appoint an international scientific committee to clean up its catalogue of pictures attributed to Goya. Two years ago the English art scholar Juliet Wilson-Bareau claimed two other well-known Goyas shown in the Prado, El Coloso, (The Colossus) and La Lechera de Burdeos (The Milkmaid of Bordeaux), were painted by someone else.

The Prado said yesterday that all of its holdings were subject to constant study and investigation but that Mr Junquera's work was "insufficiently complete or conclusive to cast doubt on the attribution to Goya of the Black Paintings." The museum denied suggestions that it had tried to block publication of Mr Junquera's book.

Manuela Mena, a senior curator and the museum's Goya expert, said: "Of course there is documentation to confirm that the Black Paintings are by Goya." Ms Wilson-Bareau said she would reserve judgement about the claims until she had read Mr Junquera's book.

The 14 works given pride of place in the Prado were originally painted on the walls of the ground and first floors of La Quinta del Sordo, a house that Goya bought in 1819. He bequeathed it to his grandson Mariano in 1823, before quitting Spain to spend his last years in Bordeaux. The frightening works are thought to bear witness to the artist's mounting despair, blamed by experts variously on Goya's deafness, depression, or gloom over the harrowing aftermath of Spain's wars against Napoleon.

But Mr Junquera says the artist's house had only one floor: the upper storey planned by Goya was added only after his death. An inventory drawn up on the occasion of Mariano's marriage in 1830 makes no mention of an upper floor, or of the paintings, and describes the downstairs rooms as "covered with a muslin lining," Mr Junquera claims. He adds that the catalogue of Goya's possessions that mentions the works for the first time – the Brugada Inventory, supposedly written in 1828 – was actually written decades later, since it contains words and expressions not used in Goya's day.

Goya's grandson Mariano sold the house in 1859 to Segundo de Colmenares, assuring his buyer that the wall paintings were his grandfather's. Mr Junquera calls this a "marketing operation" designed to boost the selling price.

"Mariano was a fraudster and a spendthrift, desperate for funds after frittering away his inheritance," Mr Junquera told The Independent. "He had difficulty in selling the house, so he said his grandfather had decorated the walls. That's when the whole saga began."

But surely the Black Paintings mark a continuity of style with Goya's previous work? "They are too similar," Mr Junquera responds. "Goya never repeated himself. It's as though a lesser hand inspired by Goya re-elaborated his work in homage to the artist.

"The similarities that tempt us into thinking the works must be authentic actually arouse our suspicions. These are not spontaneous works but planned and calculated."

Mr Junquera started his research after the British publisher commissioned him to write a biography of the artist for the general public. "But I found contradictions, so I went back to the original documents." La Quinta del Sordo was later bought by the Baron d'Erlanger in 1873 to save the paintings.

He detached the works and sent them to Paris to show them at the World Fair of 1878. They were pushed into a side passage, Mr Junquera says, and were offered to the Louvre, which turned them down. The works were acquired by the Prado in 1881. La Quinta del Sordo was demolished in the late 1890s.

Experts have challenged some 150 of Goya's prodigious output of about 500 paintings down the years but until now no one has questioned the Black Paintings. The artist worked for 60 years at astonishing speed, often finishing a portrait in one sitting. During his lifetime, countless copies of his portraits of royalty and aristocrats acquired by grand families were passed off as genuine and many copied openly by aficionados in the Prado are suspected to have turned up on the market as originals.

The New York Metropolitan now attributes to other artists three of its "Goyas" – including a copy of Las Majas en el Balcon (The Girls on the Balcony), the original of which is privately owned in Switzerland.

Was it them?

RODIN (1840-1917)

Rodin's mistress, Camille Claudel, was deeply involved in the French sculptor's work. Les Portes de l'Enfer, in the Musée Rodin in Paris, marks the start of their close collaboration. It is almost impossible to assess the extent of her influence. "She was certainly doing some of the sketches. And from the sketch to the whole making of a sculpture, there is not much distance," said the Louvre's Marie Martine Dubreuil.

FRAGONARD (1732-1806)

Jean-Honoré Fragonard's Le Baiser (The Kiss), which hangs in St Petersburg's Hermitage, was attributed in the 1980s to his sister-in-law and pupil, Marguerite Gérard.

REMBRANDT (1606-1669)

Rembrandt's The Good Samaritan in Amsterdam's Rembrandthuis, has been reattributed to one of his students, Constantijn van Renesse. The Rembrandt Research Committee claims that most works attributed to Rembrandt were in fact collaborative studio pieces.

RUBENS (1577-1640)

The contribution of Ruben's students to his work was so important that the historian Jacob Burckhardt classified his paintings in to six groups ranging from pieces completely by his own hand to workshop pieces with minimal contribution from the master. Samson and Delilah, which hangs in the National Gallery, is one of several attributions that are disputed.

JAN VAN EYCK (c.1390- 1441)

Debates over paintings by Van Eyck have raged for years. Debate over the input of his brother in his famous altarpiece in Ghent still divides some scholars.