A renowned art expert claims a painting said be the work of Raphael and the subject of a multi-million pound lottery grant, may not be authentic.
The painting is thought to have been completed in Florence in about 1507-8
The Madonna of the Pinks is owned by the Duke of Northumberland, but has been on loan to the National Gallery since 1992.
The Duke wants to sell the painting to the J Paul Getty Museum in Los Angeles for £32m.
The National Gallery has offered a compensatory amount of £21m, which it believes will be enough to keep the painting in the UK.
And the Heritage Lottery Fund has awarded £11.5m to the gallery to help it bid to keep the Raphael in Britain.
But James Beck, professor of art history at Columbia University in New York, thinks there is only a "one in 1,000 chance" of the painting being a genuine Raphael.
He said it should be valued at about £500,000 at best.
Professor Beck, who has written books and essays on Raphael and the renaissance era, said the technique used to paint the figure's hands, feet and face is not that recognised as being by Raphael.
He believes the painting may have been done by a Flemish copyist or someone employed in Raphael's workshop.
The painting is the subject of an export ban imposed by Culture Secretary Tessa Jowell because of "complex issues" over compensating the Duke.
The painting, which was initially authenticated by a National Gallery expert, is considered to be one of the most cherished small images of the Madonna and the Christ Child from the Italian Renaissance.
It is thought to have been completed in Florence in about 1507-8, just before Raphael left to start work in Rome.
It was bought by the Duke of Northumberland's family in 1853.
A spokesman for the Duke said: "As far as we are concerned we have proved it is a Raphael.
"There will always be an expert who you can bring out and who will say that black is white.
"We have justified and are confident the painting is by Raphael and that is the way we are going to go forward."