For people who buy, sell or collect old art, the hope of unearthing a new work by a big name is a motivating dream. And names don’t come much bigger than that of Leonardo da Vinci.

For eight years, the Canadian collector Peter Silverman, who lives in Paris, has been trying to convince the art world that a drawing of an aristocratic young woman he bought for around $20,000 is a long-lost masterpiece by Leonardo, potentially worth as much as $150 million. Now the controversy surrounding the drawing, “La Bella Principessa,” has taken a new turn. Shaun Greenhalgh, the notorious British art forger who is thought to have created fakes that spanned centuries of art history, has declared it to be his work.

Mr. Greenhalgh says the subject was not an Italian noblewoman, but a check-out girl named Alison who worked at a supermarket in Bolton outside Manchester in northwest England.

That sensational claim emerged on Nov. 29 in an article in The Sunday Times by the art critic Waldemar Januszczak, who is part of a consortium that has just published a memoir by Mr. Greenhalgh, “A Forger’s Tale.” Mr. Greenhalgh, who in 2007 was sentenced to four years and eight months in prison on forgery-related charges, was responsible for a number of well-documented fakes, including a Gauguin sculpture of a faun bought by the Art Institute of Chicago and an Egyptian alabaster sculpture of a princess purchased by the Bolton Museum. Now he has added “La Bella Principessa” to the list.

Mr. Januszczak says Mr. Greenhalgh was about 20 years old when he made “La Principessa,” while he was working at the supermarket in the late 1970s.

“To draw her he says he bought an old land deed that had been written on vellum, and finding the ‘good’ side to be too ink-stained to use, turned it over and drew on the rough side instead, as Leonardo would never have done,” Mr. Januszczak wrote in The Sunday Times.

Mr. Greenhalgh was unavailable to comment.

“It’s ludicrous and absurd,” Mr. Silverman said in a telephone interview. “It’s shameless that an art historian should stoop to that level to promote his book.”

Mr. Silverman said he would pay Mr. Greenhalgh 10,000 pounds, about $15,000, if he could reproduce “La Bella Principessa” on vellum in front of a committee of experts. “And he goes back to jail where he belongs if he doesn’t,” he said.

The story of “La Bella Principessa” and its attribution started in January 1998, when Kate Ganz, an art dealer in New York, bought a hea·or $21,850 with fees at Christie’s in New York. Executed in pen, ink, chalk and watercolor on vellum, it was cataloged by Christie’s as “early 19th century, possibly German.”

Nine years later, in January 2007, Ms. Ganz sold the drawing, which she described as “based on a number of paintings by Leonardo da Vinci and may have been made by a German artist studying in Italy,” to Mr. Silverman for the original purchase price, minus a dealer’s discount.

In June 2008, Lumière Technology, a company in Paris that specializes in digital scanning, announced that its analysis had determined that the portrait was by Leonardo. The work was subsequently valued at 100 million by the London art dealer Dickinson, who offered it for sale to selected clients. Timothy Clifford, a director of the National Galleries of Scotland from 1984 to 2006 who joined Dickinson as an adviser in 2007, declared the drawing a genuine Leonardo in an article in The Times of London in October 2009.

The attribution has also been endorsed by a half-dozen Leonardo scholars. Martin Kemp, an emeritus professor in art history at Oxford University who began researching the work in 2005, was an early convert, identifying the sitter as Bianca Giovanna Sforza, the illegitimate daughter of Ludovico Sforza, the duke of Milan from 1494 to 1499.

Research by Mr. Kemp and Pascal Cotte, a co-founder of Lumière, suggested that the drawing had been removed from a Sforza family album, now in the National Library in Warsaw, made to celebrate the 1496 marriage of Bianca to the Milanese military commander Galeazzo Sanseverino.

Those developments prompted the owner of the drawing when Christie’s sold it to Ms. Ganz, Jeanne Marchig, the widow of the artist and restorer Giannino Marchig, to sue Christie’s for breaches of fiduciary duty and of warranty, as well as negligence and negligent misrepresentation, claiming they should have known it was a Leonardo.

The suit was rejected by a United States appeals court in 2011 on the grounds that the statute of limitations had expired. In a separate suit against Christie’s, concerning the loss of the drawing’s frame, Ms. Marchig stated that the drawing had been in her husband’s collection by 1955, long before Mr. Greenhalgh is supposed to have made it.

As a Leonardo, “La Bella Principessa” has yet to win over the art establishment. The work has not been shown in any major national museum, and it was not included in the landmark exhibition “Leonardo da Vinci: Painter at the Court of Milan” at the National Gallery in London from November 2011 to February 2012.

That show did, however, include a recently rediscovered Leonardo, “Christ as Salvator Mundi,” circa 1499, which was bought in 2013 by the Russian collector Dmitry E. Rybolovlev for $127.5 million from Yves Bouvier, a Swiss businessman and art dealer.

Mr. Rybolovlev sued Mr. Bouvier after discovering that he had paid the sellers, the New York dealers Alexander Parish and Robert Simon, between $75 million and $80 million for the work.

Last Tuesday, meanwhile, Kasia Pisarek, an independent art historian who specializes in attribution, became the latest scholar to make a case against a Leonardo “Principessa.” In a paper presented at a conference in London, she ran down a checklist of what she termed “inconsistencies” — the lack of any documentation or copies, the presence of just three stitch-holes in the side of the vellum sheet ( the Sforza volume in Warsaw has five) and what she saw as anatomically incorrect quality of the drawing itself.

Her conclusion was that “the present attribution to Leonardo must be deemed unreliable.”

The drawing itself is at the Geneva Freeport storage warehouse and is not for sale, Mr. Silverman said, who added that in 2012 he had been offered $60 million for it but had rejected the offer. “I want the drawing to be shown all over the world so that people can decide for themselves,” he said. “But the experts who won’t accept it have refused to see it. Bureaucrats don’t like to take a chance. They’re afraid of controversy.”

By various accounts, then, it would seem that “La Bella Principessa” is either a real Leonardo worth tens of millions; a 19th-century Italian Renaissance style drawing worth tens of thousands; or a modern fake worth hardly anything at all.

But adding characters like Shaun Greenhalgh and Alison from the supermarket into the mix will liven up the debate — and make it ever more difficult to discern what the scientifically minded Leonardo would have recognized as the truth.