David Feldman, a leading stamp dealer who is vice president of the foundation [...] says art experts like Mr. Kemp, who has played a part in attributing other works to Leonardo, fear technological advances will erode their power to assess paintings.

“There is no easy way to get recognition and acceptance from the art world, particularly when connoisseurship in the traditional way is being challenged,” Mr. Feldman said.

Mr. Kemp responds that science is useful to highlight a forgery, by discovering pigments or other materials that weren’t available during an artist’s lifetime, but not to prove authenticity, which requires expertise and visual interpretation. He believes the painting’s style is too heavy-handed to be by Leonardo.

Although there has been no evidence to prove the picture isn’t by Leonardo, the foundation’s efforts to use science for a positive attribution have faced a series of obstacles.

Carbon dating only has been able to show the canvas of the portrait was produced between 1492 and 1652, a wide date range that doesn’t rule out the possibility that it is a later copy by somebody else.

The foundation also asked Pascal Cotte of Paris-based Lumiere Technology to investigate the painting. Mr. Cotte’s firm has pioneered a process called multispectral digitization, which reveals original colors of a painting and can pick out preparatory drawings beneath the painted surface.

The foundation, in a 320-page book on the work published in 2012, said Mr. Cotte, who also has done studies on the Mona Lisa in the Louvre, “confirms that the artist was likely the same for both paintings.”

Now, on its website, the foundation makes a more modest claim: Mr. Cotte’s analysis fails to show any reason why the painting “was not by Leonardo.”

Mr. Feldman said Mr. Cotte signed off on the content in the book but later changed his mind. The foundation believes Mr. Kemp, the Oxford scholar, put pressure on Mr. Cotte to do so, he said. Mr. Kemp denied any involvement in the matter.

Mr. Cotte didn’t respond to requests for comment. A statement posted on Lumiere Technology’s website warns of “flashy” and “ambiguous” use of its work to “back up risky conclusions,” although it didn’t give details.