A Florentine art collector claims that The Continence of Scipio at Christ Church, Oxford, attributed to Van Dyck, is instead a painting by Rubens.
Angiolo Magnelli offered evidence in Rome last week that the real Continence, painted for the Duke of Buckingham in 1620-21, is a work he bought from a London dealer in 1970.
He spent five years studying the Christ Church painting, and decided that it was a Rubens depicting King Darius III surrendering to Alexander.
The sinister tone of the Oxford painting is incompatible with the positive theme of Scipio's refusal to take advantage of a virgin after his victory over the Carthaginians, according to Mr Magnelli.
The painting also has spiral columns that are eastern rather than Roman and which Rubens used in many other paintings.
Mr Magnelli's painting shows a positive atmosphere more in harmony with the Continence story. There is also a carpet with a design that was a hallmark of Van Dyck.
The Oxford painting's history is unclear before 1809, when it was bequeathed to the college by Lord Frederick Campbell. Later experts attributed it to the "school of Rubens", or said that it was "improperly attributed to Van Dyck".
But Christ Church is adamant it has a real Van Dyck. The painting was included in the painter's exhibition at the Royal Academy last September.
Christopher Baker, the college's assistant curator, said: "I am not in a position to comment on the other painting. But in the view of all the Van Dyck experts in Britain, ours is a Van Dyck. The cataloguer of the exhibition at the Royal Academy was the director of the Ashmolean. The view he took was that it is a Van Dyck. No doubt the gentleman in Florence has an interesting painting, but the views of the experts here are not controversial."
Mr Magnelli said he was refused a fair hearing by the college. "I'm certain I'm right," he said.