The history of the Ghent Altarpiece – a painting also known as The Adoration of the Mystic Lamb, or in Dutch, Lam Gods – is about as rich and thrilling as it gets for late medieval paintings.

Since it was completed by brothers Hubert and Jan Van Eyck ​almost 600 years ago, it has been broken down, reassembled, forged, censored, sold by corrupt clergy, stolen by Napoleon, returned, repeatedly stolen during the Second World War – when it was coveted by Hitler – but, miraculously, most of it still remains in St Bavo’s Cathedral in Ghent, Belgium.

But the painting’s ability to generate headlines is far from over. A $2.4m (£1.84m) restoration of the altarpiece, begun in October 2012, is nearing completion and has led to the discovery that parts of it, including the eponymous lamb’s face, were substantially overpainted around 100 years after it was first unveiled in 1432.

The restoration work has revealed and revitalised the face of the lamb as it was first painted, and the results are… unexpected.

20 best Van Gogh paintings Show all 20 1 /20 20 best Van Gogh paintings 20 best Van Gogh paintings Sunflowers 1886, oil on canvas. The mysterious, so-called 'Lausanne' Sunflowers – the first Arles Sunflowers painting – with turquoise background. It was created in the summer of 1886 at Van Gogh's studio at Arles, north of Marseille in Provence. It has not been show to the public since 1948. Public domain 20 best Van Gogh paintings The Starry Night 1889, oil on canvas. This painting depicts the view from the window of Van Gogh's asylum room of Saint-Remy-de-Provence. It is one of the most recognised paintings in the history of Western culture. 20 best Van Gogh paintings Irises 1889, oil on canvas. The most recognised of several iris paintings made by Van Gogh at the Saint Paul-de-Mausole asylum, in the last year before his death in 1890. 20 best Van Gogh paintings Self-Portrait 1889, oil on canvas. Van Gogh painted over 43 self-portraits, paintings and drawings over a period of 10 years. He wrote to his sister: 'I am looking for a deeper likeness than that obtained by a photographer' and to his brother: 'People say, and I am willing to believe it, that it is hard to know yourself. But it is not easy to paint yourself either. The portraits painted by Rembrandt are more like a revelation.' 20 best Van Gogh paintings At Eternity’s Gate Oil on canvas, 1890. This painting was completed in early May, when Van Gogh was suffering a severe relapse of poor health – two months before his death. It is based on a pencil drawing, 'Worn Out', from a series of studies he made of a war veteran in 1882. 20 best Van Gogh paintings Cafe Terrace at Night Oil on canvas, 1888. This work was the first in a trilogy of paintings that featured starlit skies, completed over two years. Van Gogh was enthusiastic about the painting and wrote to his sister: 'Here you have a night picture without any black in it, done with nothing but beautiful blue and violet and green, and in these surroundings the lighted square acquires a pale sulphur and greenish citron-yellow colour. It amuses me enormously to paint the night right on the spot... I find satisfaction in painting things immediately.' 20 best Van Gogh paintings The Night Cafe 1888, oil on canvas. The five customers in the scene have described as 'three drunks and derelicts in a large public room... huddled down in sleep or stupor.' Van Gogh told his brother that the cafe owner, Ginoux, had taken so much money it was time to take his revenge by painting the place. 20 best Van Gogh paintings Bedroom in Arles Oil on canvas, 1888. Three authentic versions of this painting were made, easily discernible from the different paintings on the right wall. 20 best Van Gogh paintings Van Gogh's Chair Oil on canvas, 1888. This painting is currently at the National Gallery in London. It was completed after Van Gogh moved from the Hotel Carrel to the Cafe de la Gare at Arles, south France. 20 best Van Gogh paintings The Potato Eaters 1885, oil on canvas. Van Gogh chose this deliberately tricky composition to prove himself as a figure painter, wanting to show the way 'they have tilled the earth themselves with these hands they are putting in the dish... that they have thus honestly earned their food'. 20 best Van Gogh paintings Road with Cypress and Star 1890, oil on canvas. Also known as Country Road, this was the last painting Van Gogh made in Saint Remy, It is part of the large Van Gogh collection at the Kroller-Muller Museum at Otterlo in the Netherlands. 20 best Van Gogh paintings Starry Night Over the Rhone 1888, oil on canvas. Another of Van Gogh's paintings of Arles at nighttime – painted at a spot on the bank of the Rhone a few minutes walk from the Yellow House, which he was renting at the time. 20 best Van Gogh paintings The Pink Peach Tree 1888, oil on canvas. In a short space of time during his first weeks at Arles, Van Gogh painted some fifteen orchards, hoping they would sell well because his subject, the glorious blossoms of the flowering trees, had the power to 'cheer everyone up'. 20 best Van Gogh paintings Self Portrait with a Grey Felt Hat 1887, oil on canvas. This self portrait is slightly unusual because Van Gogh's face is angled to face straight towards the observer. Most of them depicted the face as it appeared in the mirror he used, so the right side seen in the image was actually the left side of his face. 20 best Van Gogh paintings Almond Blossoms 1888 - 1890, oil on canvas. Van Gogh loved flowering trees, which to him symbolised hope and new life. This painting was created to celebrate the birth of his nephew and namesake, the son of his brother Theo and sister-in-law Jo. 20 best Van Gogh paintings Woman Sewing This was part of Van Gogh's preparation for The Potato Eaters, where he attempted to paint a figure against the light. He struggled, telling his brother Theo: 'It's a difficult effect', yet he succeeded in a strong contrast with the almost-black head of the woman and the use of white for the window, which focuses the gaze mainly on the woman's hands. 20 best Van Gogh paintings Peasant Woman Against a Background of Wheat Oil on canvas, 1890. This painting, one of several versions by Van Gogh, has exchanged hands several times and was bought by Stephen Wynn in 1997 for $47.5 million. Seven years later, it was sold to American investor Steven A Cohen for more than $100 million. 20 best Van Gogh paintings Two Cypresses Oil on canvas, 1889. In the cypress trees of Saint Remy, Van Gogh found beauty "as regards lines and proportions, like an Egyptian obelisk". He was enraptured but also challenged: 'It's the dark patch in a sun-drenched landscape, but it's one of the most interesting dark notes, the most difficult to hit off exactly that I can imagine'. 20 best Van Gogh paintings The Red Vineyard 1888, oil on canvas. Art historians believe this is the only Van Gogh painting he managed to sell during his lifetime, and has been listed among his major works. It was sold in Brussels for 400 francs (about £3,000) to Anna Boch, an impressionist painter and art collector from Belgium, who was the sister of Van Gogh's friend Eugene Boch. 20 best Van Gogh paintings Wheatfield with Crows 1890, oil on canvas.Commonly regarded as Van Gogh's final painting (with some doubt from art historians due to lack of historical records). Around 10 July Van Gogh wrote to his brother Theo and wife Jo Bonger to say he had painted another three large canvases since visiting them in Paris the week before. Two of them, he said, expressed sadness and what he called 'extreme loneliness', but he also believed them to show the healthy and fortifying nature of the countryside.

Instead of the more biologically accurate depiction of the lamb’s face with its eyes on either side of its head – as they would appear in real life – the newly restored lamb’s face is less life-like and more “humanoid”, as some art critics have put it, with the two eyes on the front of the face, more like a cat or a person. It also has a noticeably pinker nose and lips.

The painting as it looked for the majority of its existence, before the restoration (Sint-Baafskathedraal Gent © Lukasweb.be-Art in Flanders vzw, foto KIK-IRPA)

Although the latest phase in the restoration was completed late in December last year, before and after photographs of the restored animal were posted to Twitter on Monday. A tweet by Fr A Schrenk (@frajds) has racked up more than 60,000 likes in 24 hours.

The sheep’s new face has now been compared to Zoolander’s “blue steel” pouting pose, the critically-panned Cats movie, the statue of footballer Christiano Ronaldo removed from Madeira airport, and even Mr Bean’s Whistler’s Mother reimagining.

It has also been likened to the amateur overpainting of a fresco of Jesus Christ at a church in the village of Borja, Spain, in 2012.

Of course, this is par for the course on Twitter. The process behind the restoration is not really comparable to the horror which unfolded in Spain.

Journalists at Artnet News suggested the restored sheep had “yielded shocking revelations” including the animal’s “humanoid face”.

The rejuvenated face of Van Eyck’s Adoration of the Mystic Lamb. Removal of 16th century overpainting has revealed a visage closer to that which the artist intended (Sint-Baafskathedraal Gent © Lukasweb.be-Art in Flanders vzw, foto KIK-IRPA) (Sint-Baafskathedraal Gent)

Belgium’s Royal Institute for Cultural Heritage, which is behind the restoration, is celebrating the work carried out by its team.

The institute said the non-original overpainting was “introduced in the mid-16th century to adapt the painting to the fashion of the time and to mask some local damage.”

“The underlying original painting by the Van Eycks turned out to be extremely well preserved, with only around a five per cent loss of paint,” a statement said. “This was uncovered in its entirety, and the old damage was replenished by meticulous retouches.”

The statement added: “There are no words to express the result. Liberated from the thick layers of yellow varnish and the coarser overpaints, we can discover the Van Eycks’ sublime virtuosity in abundance.”

Hélène Dubois, the head of the restoration project, quoted by The Art Newspaper in December last year, said: “This overpainting had been done so early on and following the shapes of the original, with very similar pigments that had also aged in a similar way, that it was not actually visible on the technical documentation when the altarpiece first came in for treatment.

“Nothing like this had ever been observed in early Netherlandish painting.”

She said the discovery came as “a shock for everybody – for us, for the church, for all the scholars, for the international committee following this project.”

Speaking to The Independent, art historian Dr Bendor Grosvenor said deciding to remove such old overpainting, done nearer to the time the painting was first completed than to now, could provoke debate among scholars.

He said: “There’s always a debate as to whether a bit of repainting in a picture like that is something you should preserve as part of the painting’s historical journey, or whether it’s something that is an abomination – an affront to the original artist which should be removed.

“In the case of the mystery sheep, if the overpaint had been put on 500 years ago then I think actually there’s quite a strong argument for leaving it on, not least because with modern technology you can now get a very good idea of what Van Eyck had intended beneath it. You could use digital infrared and all that stuff. It’s possible in a way to have your cake and eat it with a case like this.”

“If the sheep had been added in 1974 by an inept restorer who thought they could improve on the handiwork of one of the world’s greatest artists, then I think they would say, ‘yeah, of course we should remove that, it was ridiculous to put it on in the first place’.”

The painting undergoing restoration work in Ghent (KIK-IRPA, Brussel)

Koenraad Jonckheere, of Ghent University, said: “Conservation-restoration campaigns and the associated multidisciplinary research often raise important discoveries. The collaboration of conservators, chemists and art historians teaming up around objects allows for new insights into how a masterpiece was made, how it changed throughout history, and on the original intention of the artist.

He said it appeared extensive layers of opaque overpaint had been applied in the middle of the 16th century.

“The aim seems to have been to hide abrasions, local damage and old restorations, i.e. to make the altarpiece look as if it was perfectly preserved. This ‘renovation’ respected the original contours of the draperies and architecture although the original colors were slightly modified, the shapes simplified.”