A drawing of a trawler at sea watched by a matchstick couple with a dog on a beach caught the eye of one of our readers in a sale in Yorkshire last year. A successful investor with limited knowledge of art, he was attracted by the clear signature, LS Lowry, and dating of 1960. The drawing was fully catalogued (that is, the auctioneer had printed the artist’s name in full, denoting that they believed the drawing was authentic). The auction’s terms and conditions stated that, if it could be proven otherwise within 14 days of the sale, they would give the buyer their money back.

Another attraction was the estimate – £5,000 to £8,000 did not seem too demanding for a characterful drawing by one of the great British artists of the 20th century. For the sale, he decided to bid to the high estimate, but smelt a rat when the hammer came down at just £3,000. He was the winner; but was he a lucky one? The fact that no one else seems to have bid on the picture worried him.

A friend then advised him to take the drawing to the Crane Kalman Gallery in London. The late Andras Kalman knew Lowry better than anyone, and the gallery is now run by Robin Light and two of Kalman’s children.

The gallery has an extensive archive on Lowry, second only to the archive of the Lefevre Gallery, Lowry’s dealer for many years, which is housed in Tate Britain. The other main research resource is the Lowry Centre in Salford, but they have steered clear of authenticating works for fear of legal repercussions. Authority on Lowry now rests with a medley of dealers and auctioneers, whose opinions can differ – an ideal situation for forgers. A catalogue raisonné of an estimated 1,500 paintings is in progress, but a long way off completion.

For drawings, the situation is more complex. Lowry may have made as many as 8,000 drawings, says Light. They are less consistent than the paintings, and no one has begun to think about a catalogue raisonné. They are not studies for paintings but independent artworks and highly valued. Since November 2010, eight drawings have sold for over £100,000 at auction, one for over £200,000.

When Light first saw the trawler drawing he did not dismiss it but asked who had owned it previously and what its exhibition history was; provenance is especially important with an artist as faked as Lowry. The auction house came up with a couple of surnames that didn’t ring any bells. Light then called London auctioneer Jonathan Horwich, who has probably sold and rejected more Lowry drawings than anyone during his long career at Christie’s and latterly Bonhams.

“I turn away more fake drawings than paintings,” says Horwich. “This was a tricky one,” he adds. “Fake drawings are more difficult to spot than paintings. But in the end I thought on qualitative grounds the drawing wasn’t confident enough.”

A few days later, Light was offered a book of Lowry drawings published in 1963 while the artist was alive, with a forward by the critic Mervyn Levy. In it was a drawing of the same composition as the one from the Yorkshire auction, with subtle differences in detail and treatment. It seemed clear now that the doubtful Yorkshire drawing was a copy of sorts.

It is notable that both are dated in the early Sixties, when Lowry began to develop a more spirited style of drawing, less tightly composed and controlled than his earlier work, which is harder to fake. “They look easy to copy, but they’re not,” says Bill Clark, a Lowry dealer in Cheshire, who says he turns away eight out of every 10 Lowrys offered to him. “Fake boat drawings are the most difficult to spot because there are a lot of variants.”

“If you compare the two you see that the copy is not so animated,” says Light. “The dog and the sea are less animated, the tanker looks stationary, the drawing is flatter, lazier.”

When the Yorkshire auctioneer received the evidence, he refunded our reader, though this doesn’t always happen, says Light, stressing that it’s not just Lowry but other popular artists like Mary Fedden and Alfred Wallis whose fakes are sold in smaller auctions. Buyers need to be careful, buy from experienced galleries and auction rooms, take advice from experts and, above all, always look for a watertight provenance, he says. Sellers, especially smaller auctioneers, should also do their due diligence. This situation occurs all too often.