A painting thought to be by Titian, which will be included in a touring exhibition to begin at the National Gallery in London next March, has been dismissed as a “dud” by a leading art historian.

Professor Charles Hope, a Titian expert, told the Observer that The Danaë, an early 1550s picture on a mythological subject, is “a later pastiche” and should be downgraded from being attributed to the 16th-century Italian master to that of a minor hand.

The painting, which hangs in Apsley House, home of the first Duke of Wellington, depicts a princess called Danaë being seduced by Jupiter, the Roman god, disguised as a shower of golden rain, with an old nursemaid by her side. In August, a picture in the same collection, Orpheus Enchanting the Animals, was upgraded from coming from a little-known Titian follower, Alessandro Varotari, to Titian’s workshop.

Yesterday Hope, the former director of the Warburg Institute, a research centre associated with London University, challenged The Danaë’s attribution, suggesting that it was by a north European follower of Titian rather than an Italian artist. He argued that the “much more beautiful” version in the Prado, Madrid, is the original Titian sent to Philip II of Spain, his greatest patron, for whom he painted an epic series of large-scale mythological paintings.

However, another leading expert, Professor Paul Joannides, who believes the Apsley House picture is the version painted for the king, maintains that the radically free handling of the Prado Danaë resembles nothing else by Titian around 1550.

But comparisons with works of that date cannot be made as so little has survived, Hope said. “The one thing everybody agrees on is that the Prado picture is much more beautiful than any other [version]. So why didn’t Titian send the most beautiful one to Philip? Why does he do a less good picture for the king?”

Four years ago Hope’s doubts appeared in the scholarly Burlington Magazine, but he has spoken out now because the Apsley House painting – rather than the Prado version – will be included in next year’s touring exhibition. Hope said: “The one that’s now in the Prado, which everybody for the last four centuries believed is the real Titian, is the real Titian and the picture [he painted] for Philip II. The Apsley House one is a sub-Titian. It isn’t the picture for Philip II.”

Noting the Apsley House picture’s weaknesses, he said: “The most obvious one is the Disney caricature of an old woman. It doesn’t look like Titian’s old women at all.” He also criticised the brushwork as “inferior”.

He pointed to a contemporary description of The Danaë at the Alcázar in 1626, when only one picture of this subject attributed to Titian was in the Spanish royal collection. That account referred to “gold coins gushing down … in the form of rain. The Prado picture includes golden rain in the form of coins, while the Apsley House picture consists only of golden raindrops”.

Joannides could not be reached yesterday but, in a subsequent Burlington article, he and Miguel Falomir, director of the Prado, argued that many of those raindrops are large enough to have been remembered by that contemporary as coins. They concluded that, although the Apsley House Danaë had been ignored or dismissed by scholars until recently, a 2013 restoration revealed that its “technique and handling were wholly compatible with Titian’s work c.1550 and not with that of a later pastiche”.