THE phenomenon of Banksy, an English graffiti artist, seems to have got out of hand. Banksy, who trades heavily on his anonymity, began drawing on walls alongside streets in north London and Bristol, his hometown.

But his stencils—often of rats making mordant political jokes—have come in from the cold streets to the prosperous warmth of London galleries and auction houses. Record prices for Banksies have been repeatedly set and exceeded over the past nine months.

The rush began in February, when Sotheby's sold seven of Banksy's works in oil, enamel, acrylic and spray-paint. Bonhams took up the baton, and set the pace in April, selling Banksy's “Space Girl and Bird” for £288,000. This autumn, Bonhams has auctioned another 11 Banksies, and Bloomsbury no fewer than 21.

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Art or parlour trick?

The April record was broken at Sotheby's on October 12th, when “The Rude Lord” (pictured)—an 18th century portrait that Banksy altered to show the subject giving a middle-finger salute—fetched £322,900. At a less well publicised Sotheby's sale three days later, six Banksies sold for a total of £447,000. Sotheby's has put three more in their sale on December 12th, with a modest top estimate of £150,000-£200,000.

What is happening? Is this a significant shift in taste, with popular street art claiming a place in the mainstream, or is it a phase? It could turn out to be a bubble.

Look closely at "The Rude Lord". Banksy bought the original for £2,000 and sold it for more than 150 times that. It exhibits most of his qualities: his anarchic sense of humour, his skill as a draftsman (though he never had any training or went to art school) and the quick and easy appeal of his work.

He has become a celebrity. When he was photographed working on a piece of clever street art in late October, it made many of the London papers. Angelina Jolie and Brad Pitt think he's terrific. So does Damien Hirst, who is an enthusiastic collector.

Banksy provides expensive provender for the contemporary art market, and if this is his strength, it is also his weakness: he is very easy to copy. In April, Christie's withdrew two Banksies from sale because of doubts about their authenticity, and stories about fraudulent Banksies pop up with a regularity that disturbs the auction houses. Since Banksy does not appear in public himself, Sotheby's arranges for his work to be authenticated by Steve Lazarides, his manager (and fellow Bristolian).

This month's Art Newspaper carries a damaging story about the appearance on eBay of 25 prints that appear to carry false authentification and may have been consigned by employees at one of Banksy's galleries, Pictures on Walls. (The gallery denies that the prints are fraudulent.)

Collectors who bought Banksies when they were young and he was entertaining have noticed this year's huge price inflation and are taking their profits. The temptation is irresistible. Ralph Taylor, Sotheby's man at the cutting edge—who thinks he might have met the artist but can't be sure—says that he takes calls regularly from people saying that they have a Banksy. A few of them had picked up pieces for £300 which auctioneers estimate at £50,000, and now find they are selling at between £70,000 and £100,000.

"You can kill a market," Mr Taylor warns, though Sotheby's, who will have put 18 Banksies up for sale in 2007, seems to believe this one is robust. Banksy appears to be unimpressed. "The art world is the biggest joke going," he said in the New Yorker in May. He could prove to be one of its best jokers.

Sotheby's sale of contemporary art takes place on December 12th in New Bond Street.