In an obscure corner of the British art market, canny collectors are queuing up to buy genuine fakes, painted by the country's most famous forger.

Tom Keating startled the art world in the early 1970s when he admitted to knocking out more than 2,000 phoney pictures. A Cockney, he called them his "Sexton Blakes".

By then, most of his copies were in private collections. To compound the general frustration, he declined all pleas to list his counterfeits.

For a few years after his admission, his forgeries were next to worthless. Embarrassed dealers refused to handle them. Collectors wanted no part. But after his trial was halted because of his ill-health in 1979, the public warmed to him, believing him a charming old rogue. By 1984, when he died, his almost-Cézannes, nearly-Degas and virtual-Titians were selling for about £1,000.

The demand for real-Keatings has increased dramatically over the past couple of decades. The same paintings now fetch £10,000 to £12,000.

Keating was embittered by the lack of success for his own work and his distaste for the dealers who he thought were greedy, taking advantage of hungry painters and gullible customers. So he took his revenge.

"He thought, 'I'm as good as Rembrandt, Palmer, Renoir and all the rest of the classic painters, and I'm going to prove it'," says John Brandler, a Brentwood, Essex-based art dealer and friend of Keating.

If imitation is the highest compliment then Monet, Constable, Gainsborough, along with 100 other master painters, would have been flattered. Keating assumed their style and substance so well, his replicas are almost impossible to spot.

"He was a meticulous painter," says Brandler. "He had all the technical skills, but he was missing the one thing that kept him out of the list of the great painters he copied. He didn't have an original vision."

Keating, who was also an art restorer, specialised in painting minor works by major artists. Through the late 1950s and 1960s, he placed his frauds in auctions all through the UK, and with London dealers.

While the trade in his acknowledged forgeries is brisk, with prices on the up, the most valuable Keatings, in theory, are his undiscovered masterpieces. Hundreds, perhaps thousands, of his fakes remain in private hands. Numerous collectors are reluctant to assess their Boudins, Pissarros and Manets for fear they may find evidence of Keating's delicate touch. One of his keenest admirers, Vic Hall, from Maida Vale, London, has long appreciated the artful forger's talents. "I would gladly spend a good sum of money for one of his pieces," says Hall.

"The originals are unapproachable in price to the man in the street. But he gives us 90% of the pleasure for less than a fraction of the original's price."

Brandler says Keating's prices are likely to keep on rising. The key to determining the long-term value of an artist's output is a balance of three factors ... his fame, quality of his work and current price. Keating has the fame and the quality. The only thing out of kilter is the price. With his reputation and talent, the value of his paintings look set for a jump.

The grand faker himself is now receiving the highest compliment.

Dodgy paintings in Keating's original style, proudly bearing what-looks-like his signature, are finding their way into the market. If they manage to fool, they can claim £5,000 to £10,000.

But if uncovered they are virtually worthless, much like Keating's 20 years ago. If you can pick them up for next to nothing, they may be a better investment than an original Keating counterfeit.

Con tricks of the trade

· The work of another fantastic forger, Hans van Meegeren, is worth considering. The Dutch painter was tried in Holland after the second world war for collaborating with the enemy by selling a Vermeer to German Reichsmarschall Hermann Goring. During the trial, van Meegeren confessed that he produced the valuable painting himself, along with many others. Ten years ago his forgeries were fetching between £500 and £1,000. Today they command £25,000.

· A few years before he died, Keating thoughtfully produced a series of programmes for Channel 4, showing tricks of the tricksters. The show's two-and-a-half hours of video tapes called, Impressionism, How They Painted, is priced at £40.

· The National Gallery offers a fake-detection service. If you fear you may have bought a forgery, take it to Trafalgar Square on Wednesday afternoons for a free checkover. John Brandler says: "The only way to tell if a painting is a forgery is to look at it with an honest eye. If you are trying to fool yourself you've got the real thing for £100 discovered in a junk shop, you will convince yourself it's real. Ask 'does it ring true - do all the components belong together?' Experts at the National Gallery are incredibly knowledgeable and that comes from experience. Remember any test you know to tell the real thing, the forger knows too!"

· John Brandler 01277-222 269 www.brandler-galleries.com