Paris-based Canadian Peter Silverman says he is not the owner of the drawing which has been valued at £100m.
Silverman, who has been described as the owner in various press reports, told ATG: "I can legally say it's not mine. I bought it for a client who's going to take good care of me if it's ever sold!"
He bought it from New York dealer Kate Ganz reportedly through the Jersey-based company Downey Holdings in 2007.
He says his client is a collector of contemporary art, "independently wealthy and interested in charitable causes and animal issues", and is looking to set up a "non-profit-making foundation for multi-disciplinary Classical and Renaissance studies" near Florence, to be headed by Professor Martin Kemp.
Silverman has confirmed that the portrait will be shown in Göthenburg next March, whether a buyer comes forward in the interim or not. He is taking part in a press conference in Göthenburg to promote the exhibition - expected to attract 250,000 visitors - on October 28.
The picture first appeared at Christie's New York in January 1998, where it was catalogued as "German, early 19th century" and estimated at $12,000-16,000. It was knocked down to Ganz at $19,000 (hammer) with Silverman as an underbidder.
As to how events unfolded since then, Silverman said: "I'm not so smart, otherwise I'd have bought it in 1998.
"When I bid I thought it was good, but I was influenced by Christie's cataloguing. Not having the knowledge or courage to go all the way, it played on my mind for a year or so, then I forgot about it."
That was until he walked into Ganz's Old Master Drawings show in New York in January 2007 and saw it again.
"She must have kept it in a drawer after buying it at Christie's," he said.
"I felt it was 15th century right away - but who thinks he's about to buy a Leonardo?"
He saw it as by "somebody in the Leonardo circle" and admits he didn't notice that it was by a left-handed artist until he showed the drawing to veteran Leonard specialist Mina Gregori early in 2008, just after showing a digital image to Dr Nicholas Turner, former keeper of prints and drawings at the British Museum.
"Mina studied it for 15 minutes and said 'This is by Leonardo and I want to be the first person to write that!' She said it was painted in Milan, because of hairstyle and clothing, obviously by a Florentine artist, who was left-handed - and she knew of no one other than Leonardo who fitted the bill."
Silverman consulted Martin Kemp and Lumière Technology in Paris soon afterwards. Lumière's multispectral camera established the presence of a fingerprint on the picture which matched one on Leonardo's St Jerome in the Vatican.
Emmy-winning US TV producer Jo Ann Caplin filmed Kemp examining the work for the first time for a programme that was subsequently ditched because of the economic crisis.
Meanwhile Lumière Technology say they have been inundated by a media barrage since ATG broke the story two weeks ago.
Partner Jean Penicaut has counted 1500 press articles, TV reports on "every channel on earth", and says their website has gone from 150 to 3000 hits per day, and that even a cabbie talked to him about the Leonardo on a recent visit to London.
But Lumière were infuriated when Christie's were reported to have tentatively queried the Leonardo attribution on ABC News, as based on "unproven scientific techniques which were not available to us at the time". Meanwhile, Kate Ganz told Associated Press that "nothing that I have seen or read in the past two years has changed my mind… I do not believe that this drawing is by Leonardo da Vinci".
However, a growing body of academic opinion now believes that this is fully attributable work.
By Simon Hewitt