Italian researchers believe they have unearthed a previously unknown self-portrait by Leonardo da Vinci drawn when the artist was a young man.

The faint pencil sketch has emerged after being hidden for 500 years in one of the Renaissance genius’s most famous manuscripts, the “Codex on the Flight of Birds”, written between 1490 and 1505.

In a discovery that could have come from the script of a Dan Brown thriller, an Italian scientific journalist studying the document noticed the faint outline of a human nose hidden underneath lines of ink handwriting, It struck him as being similar in shape and drawing style to a later self-portrait of Leonardo.

It is thought that Leonardo first made the drawing in the 1480s, then later reused the same piece of paper when he came to write his codex - a Latin term for manuscript - on bird flight.

The journalist, Piero Angela, enlisted the help of art historians, Carabinieri police forensic experts and graphic artists to tease out more detail from the ghostly image.

Over months of micro-pixel work, graphic designers gradually “removed” the text by making it white instead of black, revealing the drawing beneath.

What emerged was the face of a young to middle-aged man with long hair, a short beard and a pensive gaze.

Mr Angela was struck by similarities to a famous self-portrait of Leonardo, made when the artist was an old man around 1512. The portrait, in red chalk, is kept in Turin’s Biblioteca Reale, or Royal Library.

The research team used criminal investigation techniques to digitally correlate the newly-discovered sketch with the well-known portrait.

They employed facial reconfiguration technology to age the drawing of the younger man, hollowing the cheeks, darkening the eyes and furrowing the brow.

The two portraits were so similar “that we may regard the hypothesis that the images portray the same person as reasonable”, police photo-fit experts declared.

To make doubly sure, the ageing process was reversed, with researchers using a digital “facelift” to rejuvenate the older self-portrait.

After removing the older Leonardo’s wrinkles and filling out his cheeks, the image that emerged was almost identical to the newly discovered sketch.

“When I actually tried to age the face [of the newly discovered portrait], and to put the hair and the beard of the famous self-portrait around it, a shiver ran down my spine,” said Mr Angelo. “It resembled Leonardo like a twin brother. To uncover a new Leonardo drawing was astonishing.”

The similarities were also studied by a facial reconstruction surgeon in Rome. “[He] said the two faces could well belong to the same man at different times in his life”, said Mr Angelo.

A world expert on Leonardo, Carlo Pedretti from the University of California, described the sketch as “one of the most important acquisitions in the study of Leonardo, in the study of his image, and in the study of his thought too”.

The unusual find comes after an Italian historian this week claimed to have discovered a portrait of Leonardo while studying the picture collection of a family in Acerenza in southern Italy. But other experts have dismissed the claim.

by:telegraph