Shaun Greenhalgh made the extraordinary claims in his recently published autobiography, in which he says he drew 'Portrait of a Young Fiancée' while working at his local supermarket.
“I drew the picture in 1978 when I worked at the local Coop,” wrote Greenhalgh.
“The 'sitter' was based on a girl called Sally, who worked on the checkouts.”
Greenhalgh claims he artificially aged the painting – which is currently on display at the Royal Villa of Monza, in northern Italy – by using base materials of wood, chalk and canvas that were already hundreds of years old.
“Counterfeit art is very, very common and probably easier to do than people think," Giancarlo Arù, a Rome-based expert in antique prints, told The Local.
"Entire volumes have been written about it, but it is normally slightly easier to detect in pictures from the 15th and 16th century.”
The provenance of the painting is only documented since January 1st 1998, when it was sold at Christie's auction house in New York for €20,600 as an unknown portrait from the early 19th-century German school.
But since then a number of studies have credited the work to Leonardo Da Vinci.
In 2006, forensic art examiner, Peter Paul Biro, found a partial fingerprint on the painting which he said was 'very similar' to one found on an unfinished Da Vinci work, ' St. Jerome in the Wilderness.'
Following Biro's finding, Oxford History of Art professor Martin Kemp wrote a book in 2010 outlining why he thought the work was an original piece by Da Vinci.
These findings caused the portrait's value to soar, and recent estimates of its value have put it at €150 million.
But the piece will never achieve that price after Greenhalgh's admission, according to Alù.
“As soon as there is any kind of doubt it destroys the value of a painting. Even if we spent a year and a half doing analysis and tests, we would only have a good idea if it was a forgery: an element of doubt will always remain.”
Greenhalgh was described by UK police as “the most diverse art forger known in history” when he was finally arrested for fraud in 2008.
He was caught while trying to sell a series of forged Assyrian bass reliefs to the British Museum. He has reportedly made €1.42 million by forging works of art from a great number of historical periods - the majority of which he completed in his garden shed.