Recently Tate Modern got a bit of a shock. News came winging over from Italy that an exhibition of works by Modigliani which had been shown at the Palazzo Ducale in Genoa last year had included 20 fakes.

Twenty! Were the experts at the Tate really sure that the show of portraits by Modigliani which opened in November in London were all genuine? Luckily, a voice of authority – one of the show’s co-curators – calmly assured us that all the works on display had appeared in the standard catalogue raisonnée of 1972.

So that’s all right then. Or is it? The argument seems to suggest that everyone is absolutely cast-iron certain that all the works in that catalogue were genuine. Were they? And why is a book published 50 years ago cited as the final authority anyway?

And is it not perfectly possible that the work you can see in those galleries over at the South Bank this afternoon, though seemingly identical in every way to the work described and photographed (poorly?) in a 50-year-old catalogue, may not in fact be that work at all, but merely a perfectly good copy? It is tricky though, this business of fakery. Fakes are such nasty things. Copies aren’t half so bad, are they?

Why is it OK to copy, but it’s not OK to fake? Because if you fake, you are trying to trick people into believing that something is what it is not. And for that sort of thing you can go to prison.

Whereas, if you make a copy and declare it to be a copy – and master painters have always encouraged the young pupils in their studios to make copies because that was a way of learning the craft, the tricks of the trade – that’s fine. No pretence at all. But is it quite that simple? No, of course not. Art – in common with life – is never simple.

So here’s what’s tricky about all this business of copying and fakery. What if the copy is identical to the fake? Why should it be regarded as any lesser a thing? What is more, how can anything be wrong with a fake in itself unless – or perhaps until – you know that it’s a fake? Art experts have been scratching at their balding pates over these issues for centuries, of course.

These mediators between art and its collectors, those who decide whether a painting is an authentic example of its kind or not – have often made tidy sums by declaring a painting to be true or false. Some of them have been very dishonest indeed. Some have been in it for the money. Oh dear.

The latest intriguing story in the fumy and fishy old world of copying and fakery comes from three artists who were trained in St Petersburg. Three Russian brothers who have a thriving workshop in Berlin – their names are Semjon, Michael and Eugen Posin – are making tidy sums of money by copying the most expensive painting ever to have been sold at auction.

Yes, I’m talking about Salvator Mundi, the only Leonardo da Vinci still left in private hands, which was sold in December by Christie’s New York to an anonymous buyer (which later proved to be a museum in Qatar) after a razzmatazz PR exercise. How much do one of these copies cost? According to the Art Newspaper, which broke the story, the price is $10,000 (£7,200).

They can’t paint them fast enough. We all want one, it seems. Well, enough of us to keep them very busy. Cheap at the price? Cheap for a fake? Sorry, I meant a copy. A legitimate copy. A copy shamelessly, proudly declared to be a copy.

The brothers were quick to point out that had you wanted a famous Botticelli instead (and why not, if you happened to be in the mood?), it would have been up to twice the price because copying a Botticelli is much more of a sweat.

They are making copies of things like this all the time – Caravaggios, Raphaels, Michelangelos, and always lots and lots of da Vincis, who is always a favourite. They are even planning an exhibition of their copies soon. And who buys this stuff?

They tell us: businessmen, academics, world-renowned conductors, lawyers, art historians, and even priests. Curious, eh? Why would be an art historian be so eager for a copy? And how could a priest possibly afford one? Unless he were a priest with his fingers in the collection box.

Here’s a good piece of copy about a copy. The newly expanded Royal Academy will open to great fanfare in May of this year. Works long away from home will emerge, blinking, into the light of newly refurbished gallery spaces in the old quarters once occupied by The Museum of Mankind.

Among those missing marvels will be an almost full-size, early 16th-century copy of da Vinci’s Last Supper painted by his pupil Giampietrino just a few years after the original. What a great copy that promises to be, and a very, very old copy too! Almost the real thing! No fly-by-night Russky knocked that one out in a garret in St Petersburg last week! No one will even dream of calling it a fake.

The Independent