As reported in The Telegraph:
The painting, which depicts Isabella d’Este, a Renaissance noblewoman, was found in a private collection of 400 works kept in a Swiss bank by an Italian family who asked not to be identified. It appears to be a completed, painted version of a pencil sketch drawn by Leonardo da Vinci in Mantua in the Lombardy region of northern Italy in 1499. The sketch, the apparent inspiration for the newly found work, hangs in the Louvre Museum in Paris...
Scientific tests suggest that the oil portrait is indeed the work of da Vinci, according to Carlo Pedretti, a professor emeritus of art history and an expert in Leonardo studies at the University of California, Los Angeles. “There are no doubts that the portrait is the work of Leonardo,” Prof Pedretti, a recognised expert in authenticating disputed works by da Vinci, told Corriere della Sera newspaper.
Martin Kemp, professor emeritus of the history of art at Trinity College, Oxford, and one of the world’s foremost experts on da Vinci, said if the find was authenticated it would be worth “tens of millions of pounds” because there are only 15 to 20 genuine da Vinci works in the world. But he raised doubts about whether the painting was really the work of Leonardo. The portrait found in Switzerland is painted on canvas, whereas Leonardo favoured wooden boards...More details at the link.
There are further doubts – Leonardo gave away his original sketch to the marquesa, so he would not have been able to refer to it later in order to paint a full oil version. “You can’t rule out the possibility but it seems unlikely,” Prof Kemp said. It was more likely to have been produced by one of the many artists operating in northern Italy who copied Leonardo’s works.