At his creative peak Andy Warhol was the master of mass production art, churning out prints for sale at less than princely sums.

His early Marilyn Monroe portraits would sell for less than a colour television while assistants in his “sweatshops” worked around the clock to turn out “authentic” reproductions.

But despite the glut of works circulating worldwide, the 15-minutes-of-fame Pop artist has now become the second-highest-grossing artist after Pablo Picasso.

Indeed, auction houses are discovering that the world’s art-collecting glitterati are happy to pay several million pounds for “original” Warhols.

Collectors are cashing in on the insatiable demand for the paintings, of which there are estimated to be about 100,000 worldwide.

A series of ten portraits of sports stars, including Muhammad Ali and Pelé, signed by both artist and athlete, is being offered for £14.2 million at Martin Summers Fine Art in London. Each of the ten agreed to be photographed by Warhol, who used the images to produce portraits that are to be sold by Richard Weisman, an American collector, who has plenty of other Warhols to spare.

Next month, Warhol’s 1963 Green Car Crash painting is forecast to break auction records and sell for $35 million (£17.5 million) at Christie’s in New York.

The ascent of Warhol up the ranks of top-grossing artists is undoubtedly helped by the sheer number of his prints available. He had a prodigious output, augmented by his assistants, who would massproduce thousands of reproduction prints of his originals.

The Andy Warhol Art Authentication Board, whose seal of approval is enough for leading auctioneers and dealers, will accept these “reproductions” as genuine Warhols if all the artist has contributed is his signature.

This has not stopped Russian oligarchs and others snapping up entire collections of his works. It is rumoured that one Arab sheikh recently paid £1 billion for a collection of the artist’s work.

Bloomsbury Auctions, in a move that would undoubtedly have pleased the celebrity-obsessed artist, has also decided to devote an entire sale to his prints and drawings.

Angus Maguire, its head of contemporary art, said that the art world was finally catching on to Warhol’s importance.

“With his massproduced images he took away the exclusivity of fine art and made it available to everybody on any level of understanding, from the academic to the everyday,” he said.

“The art world has been slow to catch on. Warhol is the most important international artist of the 20th century. Conceptually, he is more important than Picasso.”

James Robinson, of Martin Summers, said: “He has come hugely into fashion. There are an awful lot of people who love his art now.”

The auction record for a Warhol was set at Christie’s New York last November when Mao, 1972, sold for $17.4 million (£8.7 million). But the auction house now believes that it has, in the Green Car Crash, 1963, the most valuable Warhol painting ever to come to auction. It depicts a mundane suburban street with an overturned car in flames and a catapulted body of the driver.

Brett Gorvy, of Christie’s, said: “This is a painting that has been considered the Holy Grail by a legion of contemporary art collectors.”

The same sale, on May 16, also includes the original iconic Marilyn Monroe portrait, which experts expect to fetch around $20 million.

15 minutes

— Born in Pittsburgh in 1928, son of Czech immigrants

— Started as a window dresser and commercial illustrator. Began painting in the late 1950s

— Came to prominence in 1962 with paintings of Campbell’s soup cans, Coca-Cola bottles and Brillo soap-pad boxes

— In 1968 Valerie Solanis walked into his studio and shot him. He survived

— Died in 1987 after gall bladder surgery

Source: Britannica, Warhol Foundation, Christie’s

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