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A painting of the Mona Lisa hangs above a fireplace in a London flat in the 1960s. Is this picture not only by Leonardo da Vinci, but also an earlier version of the world famous portrait that hangs in the Louvre Museum in Paris?
Some people are convinced it is, and more than 50 years later, a bitter battle has erupted over both the ownership of the picture and the evidence about who painted it.
The so-called "Earlier Mona Lisa" is at the heart of a mystery that involves Caribbean tax havens, Swiss bank vaults, a mysterious international consortium, and the Sherlock Holmes of the art world.
So is it genuine? Who are the rightful owners? And could the portrait at the centre of this Da Vinci Code-style mystery be worth hundreds of millions of dollars?
A court case in Italy this week may finally help shed some light on the answers to these questions.
In 2012, an organisation called the Mona Lisa Foundation unveiled to the world, in a blaze of publicity, what it claimed to be a second painting of the Mona Lisa by Leonardo da Vinci.
Since the Mona Lisa is possibly the world's most famous painting, and its painter is regarded as one of history's greatest artists, such a find would turn the art world upside down.
A comparison between the two paintings displayed by the Mona Lisa Foundation at a presentation in Geneva, Switzerland
The foundation set out an array of evidence to try to back up the claim that the painting was a second, previously unknown version of the portrait. But curiously the organisation claims it doesn't own the painting.
It says the picture is owned by an unnamed international consortium. When asked about this, the foundation's general secretary, Joël Feldman, replies: "The foundation, as a matter of policy and in compliance with its obligations, does not comment on the ownership consortium."
The Earlier Mona Lisa painting unveiled by the Mona Lisa Foundation in Geneva, Switzerland
But at their home in south London, Andrew and Karen Gilbert have a different story - they say they own a 25% share in the portrait.
When they contacted the Mona Lisa Foundation after it unveiled the portrait in 2012 they claimed the organisation said it "didn't know anything about us, they weren't the owners and just tried to bat us away as someone inconvenient".
"Because we were unable to find out who the owner was, nobody was telling us anything, we didn't know how we could launch any kind of proceedings," Karen says.
Andrew and Karen Gilbert, from London, say they own a 25% share in the painting
This week has seen a dramatic development that the Gilberts believe may lead to a breakthrough in their claim.
But a claim in what? Is it possible for a near priceless Leonard da Vinci portrait to suddenly come to light?
Incredibly, that's exactly what happened with a painting called the Salvator Mundi - or Saviour of the World.
Sold for just £45 ($55) in 1958, it was bought at auction for an incredible $450m (£357m) by an anonymous buyer two years ago.
Auctioned at Christie's, Leonardo da Vinci's Salvator Mundi is the world's most expensive painting
The difference, of course, was down to the painting being authenticated by an international team of experts as a genuine Leonardo.
Could the painting dubbed the Earlier Mona Lisa by the foundation follow the same path?
"I was sceptical but intrigued," Professor Jean-Pierre Isbouts says from Santa Monica, California - he was flown to Switzerland by the foundation to view the painting.
"I walked into the vault, it was very cold in there, and I spent about two hours with that painting. But after five minutes I recognised that this had to be a Leonardo."
But it wasn't just the appearance that made the academic from Fielding Graduate University in California (whose work is recommended by the foundation) believe the portrait was genuine - it was also the historical evidence, he says.
Leonardo da Vinci pictured in Giorgio Vasari book "The Lives of the Most Excellent Italian Painters"
"Giorgio Vasari, the [16th Century] biographer of Leonardo, clearly states that Leonardo worked on the Mona Lisa for four years and then left it unfinished."
This matches the appearance of the Earlier Mona Lisa, which has an incomplete background, unlike the famous portrait that hangs in the Louvre.
Professor Isbouts also points out that historical records mention Leonardo painting the Mona Lisa for two different clients, raising the possibility that he completed two separate portraits, one for each commission.
Histograms displaying colour similarities between the two paintings at the Mona Lisa Foundation presentation in Geneva, Switzerland
He adds that scientific tests appear to back up the claim the painting is genuine.
"With the Earlier Mona Lisa the science told us a) it is from the early 16th Century, b) it is definitely a composition by Leonardo because the configuration and the composition is identical to that of the Louvre Mona Lisa. And c) the histograms [digital graphs of the colours used] show that in terms of the 'handwriting' of the painting, how he applies the paint, [it] is exactly identical."
But not everyone agrees.
"It's not the real article for a whole series of reasons," Martin Kemp, emeritus professor of art history at the University of Oxford, says.
"It really isn't a serious runner to be by Leonardo himself."
He thinks the reason Giorgio Vasari believed the Mona Lisa to be incomplete was because "Vasari's information was all Florentine", and the picture was probably completed after Leonardo had left the city of Florence.
The writer and painter Giorgio Vasari wrote about Leonardo da Vinci and the Mona Lisa in the 16th Century
And he disagrees that historical records suggest two Mona Lisas were painted.
Instead Professor Kemp says Leonardo probably never handed the portrait over to his original client, and a second person "might well have said if you finish that, I'll take it off you, as it were".
And the scientific evidence? Professor Kemp says the information offered by the Mona Lisa Foundation is only "permissive" and, while not ruling out the Earlier Mona Lisa from being by Leonardo, it certainly doesn't prove that it is.
Professor Martin Kemp is one of the world's leading authorities on Leonardo da Vinci
However, he adds "examination by infrared and other technical means, shows that it [the Louvre Mona Lisa] underwent an evolution, as all Leonardo's pictures did.
"The infrared examination of the Isleworth Madonna [as Earlier Mona Lisa is also known] is just tediously exact and is clearly the kind of drawing that's made when you're copying something rather than generating it."
Professor Isbouts, however, is critical of Professor Kemp's analysis in part because "Martin has never seen the work, and that's the beef that David and Joel [Feldman of the Mona Lisa Foundation] have, and I think it's a legitimate beef."
In response Professor Kemp replies: "The old canard that you always have to go and see it in the original, even if it is a bit of rubbish, is not sustainable, particularly with modern imaging techniques. And some of the high quality digital images, you can actually see more in them than you can see in the painting, even with a magnifying glass."
The experts may disagree about the evidence, but has all the material been presented clearly?
The BBC has seen sections of a pre-production copy of a book - written by several contributors but edited by Professor Isbouts - about the Earlier Mona Lisa, called "Leonardo Da Vinci's Mona Lisa: New Perspectives".
One of the contributors claims that in the final, published, version of the book, sections of his text seem to have been removed. He claims many of the deleted passages appear to be ones that are not helpful to the theory that the Earlier Mona Lisa is by Leonardo.
The Mona Lisa is one of the most famous artworks in the world
Professor Isbouts denied this: "I definitely edited some segments because I'm the editor. And in some cases there were passages that I don't think were scholarly defensible. I was just trying to keep the thrust of the argument intact - I certainly didn't remove anything of a contrarian nature. There were two chapters that were simply way too long."
He later emailed the BBC, having contacted the contributor to see which passages he was concerned about, and said: "We are working on the final version of the book, in hardcover, so we can still make some corrections within the available word count."
One thing supporters of the theory that the Earlier Mona Lisa is by Leonardo da Vinci have to explain is where the painting came from.
It suddenly emerged in 1913, when Hugh Blaker bought it from a manor house in Somerset.
"Blaker believed he was on to something," says Professor Robert Meyrick of Aberystwyth University who studies the life of the art dealer.
Hugh Blaker was an art dealer, critic, collector, museum curator and playwright
Despite successfully tracking down genuine paintings by artists such as Rubens, Velázquez, El Greco, Manet, Constable, and Turner, Blaker's business dealings often went badly, and he never managed to sell his Mona Lisa.
"It was like a like a catalogue of failures really, despite his best efforts," says Professor Meyrick, of the man who struggled financially in later life.
Following Blaker's death, the painting ended up in the hands of an eccentric art dealer called Henry Pulitzer - he believed the Earlier Mona Lisa was actually more impressive than its more famous counterpart in the Louvre.
Henry Pulitzer holding a copy of a book he published about the painting called "Where is the Mona Lisa?"
But Pulitzer needed some help in trying to convince the world that they were both by Leonardo.
"He started running out of money to promote it, because he wanted to prove that it was a real Leonardo da Vinci," Andrew Gilbert says.
His family knew Pulitzer, bought pictures from him, and sold him some too.
They have shown the BBC a series of documents which they say shows the family bought a 25% share of the painting in 1964. About a decade later Pulitzer locked the portrait in a Swiss bank vault, and following his death, it eventually ended up in the hands of the international consortium in 2008.
The Gilbert family claim they own a 25% share in the painting
The Mona Lisa Foundation vehemently disagrees with the Gilberts' claim, its president telling the press in July their case is "ill-founded and has no merit".
But the family have pressed on and called in the "Sherlock Holmes" of the art world to help.
"Well, I guess I don't mind it," Christopher Marinello, CEO and founder of Art Recovery International, says about his nickname.
Christopher Marinello has helped recover art worth hundreds of millions of dollars
"We've recovered about $510m (£400m) worth of art over the years. We're still heavily involved in some of the biggest cases going on right now in the art world, and we're proud of that."
But what does he think of the claim that the painting in this case might be by Leonardo da Vinci?
"I honestly don't care about any of it," he replies. "As far as I'm concerned, this is a simple matter of clients who have a contract of purchase for this painting, whatever it may be."
It's thanks to Mr Marinello that the Gilberts (who say they themselves are not sure if the portrait is a real Leonardo) began legal proceedings against the Mona Lisa Foundation in Italy while the painting was being exhibited in Florence.
Ahead of this week's court hearing, the Gilberts' lawyer, Giovanni Protti, said this is the "most tricky and interesting case I've ever worked on".
"We've had to service a writ of summons to a lot of countries all over the world."
And that work has borne fruit. After a court hearing on Tuesday, Karen Gilbert says: "The Mona Lisa Foundation declared in front of the judge that Mona Lisa Inc in Anguilla was the owner of the painting."
Anguilla is a British Overseas Territory in the Caribbean
There is nothing to suggest such an arrangement implies any wrong-doing by the foundation or the international consortium, but the Caribbean island, which is a British Overseas Territory, is known for its discreet way of doing business.
"We're chipping away at it," Karen says. "We know therefore that we're on the right track with the research that we've done."
This doesn't establish that the Gilbert family do own a share in the portrait, but it is the first time that the Mona Lisa Foundation have disclosed who the owner is.
The Louvre museum where the Mona Lisa hangs has 10 million visitors a year
In response to Tuesday's hearing, the Mona Lisa Foundation's lawyer, Marco Parducci, says: "The Mona Lisa Foundation can neither confirm nor deny the claim, by virtue of the legal obligations it has towards the owners, unless it is explicitly requested by the judicial authority."
He adds that the Gilberts' claims suggest they are motivated by "economic interest and the desire to damage the foundation" and the next hearing in March will show "that there is no case".
The legal battle will roll on, but what would Leonardo, the great renaissance scholar, make of puzzles like the Earlier Mona Lisa and the myths that surround his work, 500 years after his death?
"Oh, he would be incredibly pleased," laughs Professor Kemp. "He was interested in fame.
"He would have cringed at some of the nonsense, but the fact that his name is the best-known name in the history of culture? Yes, he'd be very pleased."
