Take a close look at this little-known painting by Francis Bacon (pictured above). It hasn't been seen in public for 45 years, but by this time next week it could have become the most expensive painting ever sold at auction in Europe.

What is it?

The painting's title is Study of Red Pope 1962. 2nd version 1971. It is a framed oil-on-canvas measuring 6.5ft by 5ft and was made, as the title suggests, in 1971.

It depicts two male figures with whom the artist was obsessed – the Pope and a man named George Dyer. It was exhibited in Paris for the first time on October 26th, 1971, in a major retrospective show of Francis Bacon's work at the "Grand Palais". It was acquired by the family of the present – unnamed – owner in 1973.

According to the catalogue notes, “against a background of naked canvas, an extraordinary outburst of controlled expression produces a maelstrom of activity, drawing the eye first to the sumptuous symphony of rounded red forms and then to the Pope at the centre of the composition whose own reflection appears in the back of the mirror and George Dyer’s in the front. Dyer’s hand is poised ready to turn off the light.”

How much is it worth?

It could, eventually, cost over €100 million. Christie’s said the painting would go under the hammer in London on Friday, October 6th, with an estimate of £60 million to £80 million. That is between €70 million and €90 million at current exchange rates.

Even if it sells at the low estimate it would become the most expensive painting ever sold at auction in Europe. If bidding reaches the top estimate – and with Bacon this is quite possible – the buyer will probably need in excess of €100 million when auction fees and exchange rates are factored in. This isn’t pre-auction hype. Bacon has form here. Over the past decade his paintings have regularly sold at auctions in London and New York for tens of millions of pounds/dollars each.

The highest price ever paid for his work was in New York in 2013 when – also at Christie's – his life-size triptych (three-panel) painting of his friend and fellow-artist Three Studies of Lucian Freud sold for $142.4 million. In the 1950s Bacon's paintings were being sold in London for, on average, £200.

Who was Francis Bacon, and wasn’t he Irish?

Up to a point. Bacon was born in Dublin in 1909 – the son of a British army officer turned racehorse trainer – and raised in Co Kildare. He ran away to England in his teens and never came back. He became a prominent artist in London in the mid-20th century, and died in 1992. After his death his estate donated his studio to the people of Ireland, and it was shipped to Dublin and reconstructed in the Hugh Lane Gallery where it is on public display. In the international art market he’s generally classified as a British artist.

Love or loath him, Bacon is widely regarded as one of the most important artists of the 20th century, and prices for his paintings have rocketed in the past decade. The late British prime minister Margaret Thatcher once famously referred to him as "that man who paints those dreadful pictures".

Who’s who in the painting?

Bacon was obsessed with the two figures depicted in Study of Red Pope 1962. 2nd version 1971 and, although he painted both on several occasions, this painting represents the first and only time that both are united in a single image.

Bacon often painted images of the Pope – sometimes very contorted and agonised – which have become known as his "Screaming Pope" paintings. His inspiration was the famous 17th century Portrait of Pope Innocent X by the Spanish artist Velazquez.

The other figure in the painting is George Dyer, an East End criminal and alcoholic who Bacon met in a pub in 1963. The two became lovers and Bacon depicted Dyer in an estimated 40 paintings.

Study of Red Pope 1962. 2nd version 1971 was made in spring 1971. Six months later, in October 1971, Dyer went to Paris with Bacon for the exhibition where the painting was to be shown for the first time. However, Dyer died from a drink and drugs overdose in his hotel bathroom 36 hours before the exhibition opened. His death was reputedly kept quiet for two days to allow Bacon to attend the opening.

So, the art world regards this as an important painting?

Is the Pope a Catholic? According to Francis Outred, chairman and head of post-war and contemporary art at Christie's, "this painting is quite simply art history" and, ahead of the auction, "gives me a shiver down my spine".

Bacon’s biographer Michael Peppiat said the artist’s Pope paintings are “a centrepiece of the whole of twentieth-century art”.

What’s the reason for the painting’s confusing title?

Bacon made at least 50 paintings featuring images of the Pope. One, dated 1962, is titled Study from Innocent X and depicted a "red pope". Bacon wanted to show this in his Paris exhibition in 1971 but had already sold it and the owner refused to lend it. So he embarked on this new version titled Study of Red Pope 1962. 2nd version 1971.

Study from Innocent X, incidentally, sold at Sotheby's, New York, in May 2007 for $52.6 million.

Christie’s Post-War and Contemporary Art Evening Auction takes place at King Street, London, on Friday, October 6th.