For almost 80 years it has been regarded as one of Francisco de Goya’s towering glories. But yesterday it was revealed that The Colossus was not painted by the Spanish master at all, but by an understudy.

After a seven-month investigation, experts at the Prado gallery in Madrid came to the reluctant conclusion that the masterpiece was probably the work of Asensio Juliá, one of Goya’s assistants. They said that the painting, which has hung in the Prado for 78 years, was “Goyaesque but not by Goya”.

Manuela Mena Marqués, head of 18th-century art at the Prado, said: “Seen with adequate light, the poor technique used in its light and colour becomes manifest, as does a marked difference between The Colossus and other masterpieces attributed to Goya.”

Doubts have surrounded the painting for years, with a series of experts claiming that Goya never painted what has long been regarded as one of his most famous works. Experts at the Prado started an investigation last year when José Luis DÍez, the gallery’s curator of 19th-century art, suggested that The Colossus was the work of Juliá after a detailed analysis of the picture. He based his claims on the discovery of the initials A. J. in one corner of the canvas.

Juliá is thought to have collaborated with Goya on the frescos of the San Antonio de la Florida church in Madrid and usually signed his work in this way. Assistants in Goya’s studio did paint copies of some of the master’s works.

Yesterday The Colossus was still attributed to Goya, but this may change to “From the school of Goya”. It was uncertain whether Juliá would be acknowleged formally for his work.

Francisco Calvo Serraller, an art expert and former Prado director, wrote in the Spanish daily El PaÍs: “We find ourselves in front of a painting which is evidently Goyaesque, by a painter near to or an admirer of Goya, but not by Goya. You can see this in a dramatic or normal way. I think what should always be normal is the truth.”

Goya experts have been split for years over the authenticity of the painting. In 2001 Juliet Bareau-Wilson, a British art historian, claimed that The Colossus and another work, The Milkmaid of Bordeaux, had been created by others. Her theory was supported by Mrs Mena, who agreed that doubts had existed for some time about both the paintings.

The Prado denied the claims at the time and many experts believe the quality of the work means that Goya must have been involved.

Nigel Glendinning, a British art historian, doubted that anyone but Goya could have painted the work. He told the Spanish newspaper ABC: “I never said it would be impossible that [someone else] might have intervened in the work of Goya, but the painting is too audacious to be by Asensio Juliá, because of the centrifugal strength of the composition and its iconic power. I hope to be able to see the study and the proofs.”

A pioneer of early 19th-century techiniques, Goya is considered to be one of the world’s first “modern” artists, with a penetrating and incisive view of humanity. The Colossus, which is dominated by the figure of a giant who bursts through the clouds to terrify villagers below, was painted during the Peninsular War against France by Britain, Spain and Portugal between 1808 and 1814. An inventory of Goya’s possessions in 1812 refers to a work called The Giant.

Experts claim the that giant represents the Spanish people emerging to expel the French invaders who occupied Spain at the time. Others say that the horrified figures of the people represent the madness of war.

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