A newly discovered painting by Mr. Van would indeed be valuable. Regarded as a revolutionary hero and one of Vietnam’s greatest painters, he died in 1954 from injuries he suffered at the Battle of Dien Bien Phu. In 1996, he was posthumously awarded the Ho Chi Minh Prize, one of the country’s highest honors. Christie’s sold the painting to an unidentified buyer for about $45,000. But Vietnamese art experts said the painting was actually an uninspired copy of “The Young Beggar,” by the Spanish artist Bartolomé Esteban Murillo. Painted in 1650, it hangs in the Louvre in Paris.

Mr. Van’s son, To Ngoc Thanh, an established painter himself, called the painting attributed to his father a fake. “I can tell you 100 percent that is not my father’s painting,” he said. “I am disgusted. In this country there is a lot of fake art. Some crook just used that trick to make money.”

Christie’s said it had completed “rigorous due diligence” and had “no basis” to question the painting’s authenticity.

Even at the country’s most prestigious museum, the Museum of Fine Arts in Hanoi, officials have long been uncertain which of their treasured paintings are copies and which are genuine.

During the war with America in the late 1960s, museum officials removed hundreds of artworks for safekeeping, in case Hanoi was bombed by the United States, and commissioned copies to replace them. Originals disappeared, copies were passed off as originals, and no one knew which was which. Asked whether the museum had since tried to sort them out, the director, Nguyen Anh Minh, only smiled.

Adding to the confusion, relatives of some prominent artists were known to certify copies as originals so they could fetch a higher price. At an auction in Hong Kong last year, Christie’s sold the paintings “Boats on the Perfume River” by To Ngoc Van for $57,000, and “A Lady of Hue” by Le Van De for $89,000. Identical paintings are hanging at the Museum of Fine Arts in Hanoi. The museum says it acquired “Boats” in 1965 and “Lady” in 1976.