A Damien Hirst original...

... that was painted by Rachel Howard. As Sotheby's prepares for a £65m auction, the backlash is building. By Andrew Johnson

Earlier this year the artist Rachel Howard sold a painting for £61,000 at an auction in New York. In May another painting by her sold for £1.5m. The difference is that the second painting had Damien Hirst's signature on it.

It is called Amphotericin B, a 1993 spot painting which, like all of Hirst's similar works, is named after medical terms. Howard, who is little-known to any but art-world cognoscenti, is most likely to have painted the picture. Although one of hundreds of artists employed by Hirst over the years, she was working in the Hirst "factory" in 1993 and is the best at the trademark spots.

"The spots I painted are shite," Hirst has said. "The best person who ever painted spots for me was Rachel. She's brilliant. The best spot painting you can have by me is one painted by Rachel."

Tomorrow another £65m is likely to pour into Hirst's considerable coffers – he is believed to be the richest artist in the world – from the auction of 223 new works at Sotheby's. Some are spot paintings, including one valued at £700,000.

But the prices Hirst commands are creating a growing backlash against him in the art world. Last week the esteemed art critic Robert Hughes dismissed his work as "absurd" and "tacky". The week before, the influential Art Newspaper revealed that £100m-worth of Hirst art works remained unsold at his main dealers, White Cube in Hoxton, a claim White Cube denied.

Don Thompson, the author of the $12m Dollar Stuffed Shark, an exposé of the contemporary art market, said that there is nothing unusual in an artist employing others to complete their works, but added that Hirst's value's are fuelled by branding rather than intrinsic merit.

"It's well understood that Hirst does this, so nobody is being misled," he said. "The spot paintings being sold are by Hirst, and can be acquired at a Sotheby's auction, so somebody will be willing to pay for it. But we're not talking about art's intrinsic worth. We're talking about branding and status. That's why people spend £5,000 on a Louis Vuitton bag."

Tim Marlow, director of exhibitions at White Cube, defended the spot paintings. "They're incredibly original and counterbalance the decline of originality in the history of painting," he said. "It's taking something that looks machine produced but is actually painted by hand. What we see is not what we see."