The wave of forgeries, the biggest ever involving a British artist, runs into millions of pounds - so great that if it continues the artist's legitimate market will be affected.
This warning comes from the Henry Moore Foundation, which in looking after the artist's estate plays a leading role in keeping check on fakes. A dozen volumes bulging with photographs of fakes line a shelf at its headquarters in Hertfordshire. The foundation keeps meticulous records on each and every fake it comes across - mostly through dealers, auctioneers and collectors seeking authentication.
Reclining figures and mother-and-child compositions are the subjects most often faked, and there are approximately 200 drawings and 100 sculptures on file. Julie Summers, deputy curator of the foundation, said: 'It's terrifying.'
All but one of the known fakes are still in circulation. Mrs Summers said: 'Collectors or dealers hope to sell them off quietly.' Even when the foundation's experts point out they are fakes, some people plead their case obsessively.
' 'Mr Moore gave this to me himself' is a classic,' Mrs Summers said. 'But he rarely gave away works to people other than family and friends.'
The fakes continue to reappear on the market. Some come to the foundation's attention time and time again. Mrs Summers said: 'The law in Britain does not permit us to do anything. Our hands are tied. We have to rely on goodwill. The French, however, can remove a signature from a fake.'
Most of the faked bronzes are maquettes, or working-studies generally no more than 10 inches high, which sell for around pounds 30,000 and cost the forger little more than pounds 500 to cast. Works in that price-bracket are obviously easier to make and sell. Mrs Summers said: 'If the forgers sell them for pounds 10,000, undercutting the true market value, they are still profiting.'
The foundation does not know where the fakes are being produced. Most seem to surface in America, France and Germany. They believe that there are several forgers worldwide.
Some fakes, she said, are loosely based on Moore and, though not copied from an exact piece, are definitely signed; others are cast from his original casts. In those cases, forgers have had to obtain an original: if they make the investment, it can easily be sold later.
She said that only one forger has really impressed them with his artistry - a drawing, based on one of Moore's coal-mining subjects, submitted to them by a dealer. It initially divided the experts, although there were weaknesses in the figures: for example, the strong, muscled backs of Moore's miners were absent. However, it was on paper that no one had known him to use before, which raised their suspicions: the final proof against it came when the dealer foolishly sent them another drawing, one of the underground shelter images. The paper and the size were identical. Some fakes, she added, are let down by the forgers not doing their homework. In a series of 14 fakes of a reclining figure, they numbered the edition incorrectly. Moore produced an edition of nine: the forger has marked them out of seven, six and four.
Mrs Summers brought out both the original 1975 maquette of a mother and child and its faked version: she gave only one of these hand-sized bronzes the white glove treatment as she placed them on the table. She explained that one of the tell-tale signs with fakes is that the base has been cast with the sculpture itself. But it was the foundry mark that gave everything away: although it shows the H Noack Foundry in Berlin, which Moore did use on occasion, for that model he went to a different foundry, Fiorini in London.
Another giveaway is that the fake is half a centimetre smaller. She said: 'When you take a cast, the bronze shrinks slightly during cooling: therefore, if you make a cast of a cast, the result will be slightly smaller and slightly less detailed. Moore's thumbprint on the mother's face, and his beautiful, textured scratching on her back are absent from the smooth fake.'
Most look entirely unconvincing - even without comparison with the real thing. Surprisingly, perhaps, many of the forgers seem to have difficulty getting the signature right.
Tracking down the owners is impossible: it would take years of research. The big league players in the art world - the leading dealers and auctioneers - automatically contact the foundation before selling a Moore. But when a fake is unmasked, Mrs Summers said, they cannot reveal the names of their clients. Her main concern is that lesser members of the trade fail to use the foundation.
She said that even when the foundation contacted a small auction house in the South-west, alerting it of a fake in its sale, it insisted it had a Moore and proceeded with the sale.
Forgers have been 'doing' Moores since his lifetime: they began as soon as his works became valuable, though success came relatively late to him. He was concerned that it would undermine his work. But there was nothing on today's scale.
In theory, his work is hard to fake because it is so well documented. The foundation has an impressive archive that records more than 5,000 drawings and 6,000 sculptures (or 1,000 individual sculptural ideas in editions of perhaps six or nine).
It urges anyone with a Moore to get in touch. There is no charge. Already, the foundation is looking at hundreds of Moores every year and most are perfectly all right. The oundation's ability to verify works should instil confidence in the market.
Philip Saunders of Trace magazine, which liaises with the police and art world in tracking down stolen works of art, said: 'I am shocked by the number of fake Moores. I find it very encouraging that such a body . . . is going about classifying whether a work is by an artist in such a conscientious way.
'I wonder how many other artists of the same calibre who don't have such foundations to keep such a diligent check are being faked.'