In May 2008, some of the world’s greatest Leonardo da Vinci experts stood around an easel in a skylit studio high above Trafalgar Square. The object they had been invited to scrutinise, in the conservation department of the National Gallery, was a painting on a panel of walnut wood. It showed a long-haired, bearded man gazing straight ahead with one hand raised in blessing, the other holding a transparent sphere.

“There’s a mixture of being excited but not getting caught up in it,” says Martin Kemp, the eminent art historian who was there that Monday. “I try to keep a gravitational pull going, saying, ‘This can’t be right.’” Yet the thrill in the room was tangible. The painting had “presence”, felt Kemp, and there was no dissent.

That day, a long-forgotten old picture was authenticated as Leonardo’s lost masterpiece, Salvator Mundi (Latin for Saviour of the World). Three years later, in November 2011, this portrait of Christ was unveiled for the first time in the National Gallery’s blockbuster Leonardo exhibition. Six years after that, it became the most expensive painting ever auctioned, when it sold at Christie’s for the stupendous sum of $450.3m (£342.1m).

Then, last month, something perplexing happened. Salvator Mundi had been purchased from Christie’s for the Louvre Abu Dhabi. Its unveiling was scheduled for 18 September – a big moment. But at the start of September, this was suddenly and mysteriously postponed. “Further details will be announced soon,” said the official statement.

Record sum … excitement at Christie’s New York as bidding neared its end on 15 November last year. Photograph: Peter Foley/EPA-EFE/Rex/Shutterstock

There has been no further announcement and my enquiries were met by just a resending of the statement. Kemp admits he’s in the dark but insists: “It isn’t a matter of cold feet.” That’s how it looks, though.

Even as the auctioneer’s hammer went down, a chorus of scepticism was creating uncertainty around Salvator Mundi. Had those experts at the National Gallery been taken for a ride? One insider summed up the situation to me bluntly: “It’s not very good.” Stories have emerged that complicate its provenance or history. Matthew Landrus, an Oxford academic, has even gone public with the claim that, far from being a Leonardo, this work was largely done by his third-rate imitator, Bernardino Luini.

But if the Louvre Abu Dhabi really has got doubts about Salvator Mundi, they will most likely be about its condition. For there really is a problem with this painting and it is there for anyone to see. If the Louvre – both its new outpost and its home in Paris, which has the most sophisticated conservation technology on Earth – has not yet spotted the issue, all its curators need to do is check out an Instagram post that materialised just after the painting’s sale last year.

Thomas Campbell, former director of New York’s Metropolitan Museum, wrote: “450 million dollars?! Hope the buyer understands conservation issues.” The accompanying picture shows Salvator Mundi in the middle of its restoration. All the previous repaints have been cleaned off to reveal an image with streaky gaps, including a sizeable few running from top to bottom. The implication was that the painting as sold by Christie’s is over-restored. When challenged, Campbell added: “Was simply remarking, as so many others have, on extensive amount of conservation.”

Cancellation … the Louvre Abu Dhabi, which was due to show the work. Photograph: AFP/Getty

In fact, the photograph was something of a bombshell, a glimpse of a painting that looks dramatically different from the restored version. Time had left Christ partly bald, with impaired eyes, yet the face was truly beautiful – smooth and harmonious but anatomically precise. It is completely different, in tone and feeling, from the smoky, ambiguous appearance of the painting today, after its full treatment by the respected restorer Dianne Dwyer Modestini.

The image in Campbell’s post was cropped and blurry but the Guardian is today publishing a high-definition version. If the scars of age are even more visible, so is the youthful beauty of Christ. He looks like just the kind of androgynous, long-haired model Leonardo loved to portray and, said his 16th-century biographer Vasari, surround himself with, in a workshop that was the Renaissance precursor to Warhol’s Factory.

It was Martin Clayton, curator of Leonardo’s drawings at the Royal Library in Windsor Castle, who suggested I check out Campbell’s post and drew my attention to the startling differences between the painting after it was cleaned and its appearance now. “Photographs seem to show that, before it was touched up, it was all Leonardo,” he says. “They show the painting mid-restoration – and it looks as if the subsequent retouching has obscured the quality of the face.” Clayton is not questioning the painting’s authenticity. He’s suggesting that a very pure Leonardo has been partly “obscured”.

I took this troubling theory to Robert Simon, the man who discovered Salvator Mundi along with two business partners. They bought this apparently insignificant picture at a Louisiana auction in 2005. Simon is passionate about Leonardo and, when he started to think this was something more than a bad copy, set about carefully researching its provenance, while bringing in Modestini to restore it. “The most important decision was not to treat this as a simple commercial decision,” he says. Instead, the work was carried out in accordance with “a very slow, prepared and not rushed plan”.

Timeworn beauty … full view of the cleaned-up painting. Photograph: Courtesy Dianne Modestini / © 2011 Salvator Mundi LLC

It paid off when they showed the partly retouched painting to Nicholas Penny, who was then about to take over as director of the National Gallery in London. “He got it. He said, ‘I think you have an interesting problem: how do you approach something that seems almost impossible?’”

Penny was right. The discovery of a previously unknown painting by Leonardo does seem “almost impossible”. Only about 20 paintings by him survive. Others are known to have been lost or destroyed, but he was never prolific. Those few existing paintings have been treasured, making the reappearance of a forgotten one even less likely.

Penny’s solution was to bring the painting to the National Gallery and set up that expert viewing. Then it was shown for the first time in the uniquely authoritative setting of the gallery’s Leonardo exhibition. In 2013, just a year after the exhibition finished, Simon and his partners sold Salvator Mundi through Sotheby’s to a middleman for a Russian art collector, who later sold it for that record-breaking price.

Yet, even before it reached the National Gallery, the painting had been worked on. Simon confirms that it was partly “in-painted” before being shown to experts, including Penny. Why didn’t he leave the painting in its raw yet beautiful state after it was stripped down? Wasn’t that an incredible object in itself?

We felt that to leave the painting 'raw' would cause viewers to focus on the losses

“The painting was powerful as it was without further treatment,” he says. “We considered leaving it, considered more limited restoration, as well as a more extensive one.” These were not casual decisions, he insists. “Part of our final decision was made with the understanding that to leave the painting ‘raw’ would inevitably cause viewers to focus on the losses and not on what survived.

“In the end, we decided to do what we felt was best for the picture. That might sound false or corny, but it was out of a profound respect for the painting itself that we felt that bringing it back to life as much as possible was the right way to go.”

Simon absolutely rejects the possibility of any “falsehood” being introduced. “I found [Campbell’s] comments both ill-informed and offensive,” he says. As for the repainting, he regards that as a loaded term. “‘Inpainting’ is the right way to describe what has occurred here – retouching restricted to areas of loss. In the restoration, no original paint was covered.”

Excessive restoration? … how the work looked at auction, when it became the world’s most expensive painting. Photograph: Reuters

That last claim doesn’t seem right, however, when it comes to the hand of Christ raised in blessing. When the painting was cleaned, it turned out Christ had two right thumbs. This is what art historians call a “pentimento” – literally, a repentance, used to mean a second thought. If the artist had such a second thought, it’s regarded as evidence that this is an original, not a copy – as why would a copier have second thoughts?

This explains why the thumb was left with its pentimento when it was shown to those experts, even though some in-painting had been done. However, by the time of its public unveiling in 2011, Christ’s hand had just a single thumb. “Both thumbs,” says Kemp of the painting’s raw state, “are rather better than the one painted by Dianne.”

So a crucial piece of evidence that Leonardo painted Salvator Mundi also suggests that its restoration has been excessive and has muffled its power. Ironically, this seems to make the work both an original and, in my view, a kind of kitsch concoction.

Kemp has a further point to make. One of the rules of all public museums, he says, “is that you don’t exhibit something that’s on the market. The National Gallery received an assurance that it was not – but if it’s owned by three dealers, that doesn’t make a great deal of sense.”

Perhaps it was institutional embarrassment about the decision to work with a group of art dealers to authenticate a Leonardo that explains why no expert at the National Gallery – nor those involved with it who have since left – would be interviewed. Perhaps not.

Was I looking at a work of genius – or a smoky imitation that turns his brilliance to mush?

An official statement said: “The National Gallery makes careful consideration before including any picture that is in private hands in an exhibition. It weighs up the advantage in including it – the benefit to the public in seeing the work, the advantage to the argument and scholarship of the exhibition as a whole. On that occasion, we felt that it would be of great interest to include Salvator Mundi in Leonardo da Vinci: Painter of the Court of Milan as a new discovery, as it was an important opportunity to test a new attribution by direct comparison with works universally accepted as Leonardo’s.”

It was indeed a haunting thing to see. Was I looking at something that was a work of genius, or a smoky imitation of Leonardo’s style that turns his brilliance to mush? Surely it would have been more true to the greatest artist who ever lived to let his timeworn masterpiece speak to us directly. Is the Louvre Abu Dhabi taking a closer look at what it has? I think it should.

• Living with Leonardo: Fifty Years of Sanity and Insanity in the Art World by Martin Kemp is published by Thames and Hudson.