It must be the dream of every art historian to see a newly discovered work by the artist who has been his life’s work. To see two in quick succession and to judge them both as genuine works by Leonardo da Vinci is truly extraordinary.

Yet the past few years have brought exactly that experience to Martin Kemp, Oxford scholar and world authority on Leonardo. They are the first important Leonardo discoveries in more than a century. “It’s absolutely remarkable,” he said. “Two works: the Salvator Mundi painting and the drawing on vellum. The last painting to come along, which won general acceptance, was the Benois Madonna in the Hermitage in St. Petersburg, and that emerged in 1909.”

Only one of them, the recently restored Salvator Mundi, is in the current Leonardo exhibition at the National Gallery in London. The other, a profile of a young woman in ink and chalks, is not. Dubbed La Bella Principessa, is not yet universally accepted as a Leonardo, despite Kemp’s conviction and that of other experts.

To coincide with the London exhibition, Oxford University Press has updated Martin Kemp’s best-selling book, Leonardo. The new edition is the first book to include the two rediscovered Leonardos.

At his home in Woodstock we talked about how a work like the Salvator Mundi is authenticated using connoisseurship, provenance research, scientific analysis, comparison with drawings (in this case two drapery studies from the Royal Collection in the show), and looking at copies.

Some scholars rely on traditional connoisseurship, but for Prof Kemp, it’s the style. “For me to get going, to really put in the work, there has to be that frisson of thinking this is not just some routine derivative work, or a forgery.” And you do get a feeling, he said, when you’ve looked at so many. Four years ago he was invited by Nicholas Penny, director of the National Gallery, to look at the painting with other Leonardo scholars — a book on the subject will be published some time in the New Year.

“Remarkably for the art world, although a number of people knew about it, it didn’t leak out,” he said. It was always thought that Leonardo had painted a Christ as Saviour of the World. A 1650 Wenceslaus Hollar etching has a rather stocky Christ in almost identical pose — direct stare, right hand raised in blessing, left holding crystalline orb. Titled ‘After Leonardo da Vinci’, most likely it was copied from the Salvator Mundi in the collection of Charles I.

“And in Leonardo’s case you have the fact that Leonardo considered that painting was a science — a scienza; not quite science as we think of it, but a systematic body of knowledge. His particular brand of looking at things, the optics of seeing and representing — it’s that totality in which you look at Leonardo.”

How a work emerges and how it is presented to scholars is incredibly important, he stressed. This brought to mind the controversy surrounding La Bella Principessa, a drawing with no known history prior to sale in 1998 at Christie’s in New York. “It emerged in a somewhat messy and inconclusive way before all the evidence was there, and significant Leonardo scholars were alienated by the process,” he said.

New evidence has emerged about this beautiful drawing of a young Milanese lady with elaborately bound pigtail. The picture matches the pages of a book (in Warsaw), even down to the stitch holes. The book, a Sforziad, is a volume eulogising Duke Ludovico’s father, Francesco.

“We have good evidence, good technical evidence, and evidence of the hairstyle et cetera that this was a Milanese princess (‘princess’ used loosely). I suggested the most likely candidate from the circle of top Sforza young women was Bianca, Ludovico’s illegitimate daughter, who was legitimised to marry the commander of the duke’s forces, Galeazzo Sanseverino.”

The wedding was in 1496, the book a gift. Prof Kemp thinks the National Gallery exhibition, which continues until February 5, “an amazing assembly of paintings with a range of supporting drawings. . . a once-in-a-lifetime opportunity”.

Would he love to have seen La Bella Principessa there?

“It would have been incredibly revealing and telling. Just as the Salvator Mundi adds a dimension to the show, this would have done so — particularly because they’ve got the closest drawing to it there: the silverpoint drawing of a woman (from Windsor). It would be lovely to see those two together.”

The 2011 edition of Leonardo (OUP, £10.99) is the first book on Leonardo to include the two new works. La Bella Principessa (Hodder, £18.99) by Martin Kemp & Pascal Cotte was published in 2010 to tell the story (thus far) of the new work.