In his final appraisal, Crichton-Stuart valued the painting at $100,000.

Christopher T. Apostle, an expert on old master paintings at Sotheby's, said he disagreed. "While all the old documents said it was a copy, when I saw the painting I realized it was high quality and from the 16th century," he told the Times.

He also said the curators from the Metropolitan Museum of Art in New York, the National Gallery in London and an independent Italian scholar were all convinced it was an original from del Sarto. On NBC's "Today" show, Mr. Apostle restated his opinion. "It's in very good condition," he said.

Shearman, who is well-connected in art circles, wrote a letter to Sotheby's in early December, saying he doubted the painting was wholly authentic.

"Without question, it comes out of his workshop," Shearman said in an interview.

He said the painting's outer layers were likely painted by one of del Sarto's assistants, while the master etched the figures beneath.

As soon as he reached his conclusion, "I told them about my findings as soon as I saw the painting in early December," he said.