The Metropolitan Museum yesterday defended the authenticity of its most expensive acquisition — a small “Madonna and Child” attributed to the early Renaissance master Duccio di Buoninsegna that was purchased for a reported $45 million. Art historian James Beck, a professor at Columbia University, claims that the painting is a fake.

In a statement, museum officials said the painting is “considered by virtually every expert in the field to be a keystone in the history of Western art. As a matter of course, the picture was carefully examined by the museum’s curators and conservators before its acquisition in 2004, and later subjected to technical examination that gives the Metropolitan no reason to doubt that this is a masterpiece of the late 13th or early 14th century.”

Mr. Beck’s argument rests on the fact that the painting places the Madonna and child behind an illusionistic parapet. While other experts consider this evidence of Duccio’s genius of innovation — Met curator Keith Christiansen has called it “the first illusionistic parapet in European art” — Mr. Beck sees it as simply an anachronism, and thus evidence of forgery.

In his view, the depiction of threedimensional space couldn’t have been done in 1300. “It took me about three months to figure out. I looked at it and said, ‘How could this happen, a hundred years of time?'” he said in an interview with The New York Sun. “Then I realized, that’s the way very good forgers work: They use one element from one period and one element from another. So that was the clue that this was not genuine.”

Mr. Beck believes that the Met, because of its desire to acquire a Duccio, didn’t apply due diligence before the purchase. “They didn’t really have a local expert,” Mr. Beck said. “They bought it on their gut feeling. And on the feeling that after all, it’s been regarded in the books as a very good painting.” He further argues that the Met did not conduct the proper lab tests of wood before making the deal.

Because the painting was in a private collection for many years, Mr. Beck said, none of the art historians who had written monographs about it had actually seen it; they had only seen photographs. “But even if they had, I wouldn’t care,” he added. “In 1492, everybody thought the earth was flat.”

A spokesman for the museum, Harold Holzer, said that the Met’s curators and conservators examined the painting before its purchase. After the purchase, it was “subjected to many technical tests and passed all of them.” He said it was also “subjected to very close examination — both of the painting itself and at the back, and at the way the panel was constructed, which helps the conservators and the experts date it to the 14th century, and identify it to Duccio and his workshop.”

Mr. Holzer declined to respond to Mr. Beck’s argument in a point-bypoint manner. “[Mr. Beck] is not an expert in the field,” Mr. Holzer said.

In the past, Mr. Beck has not shirked from courting controversy and standing up to the art establishment. He led the opposition to the restoration of Michelangelo’s Sistine Chapel ceiling and is the author of a book called “Art Restoration: The Culture, the Business, and the Scandal.”

Last year, he challenged the authenticity of another Renaissance painting — “Madonna of the Pinks,” attributed to Raphael — for which the National Gallery in London had paid a record $60 million.

His claims about the Met painting form the last chapter of his forthcoming book, called “The Crisis in Connoisseurship.” The book is set to be published next year in Italy.

To Mr. Beck, the Met’s behavior in acquiring this painting is typical of the behavior of a powerful institution. “Where are the checks and balances? Where is the transparency?” he said. “If I were the trustees, I would want a report.”