It had been a very good auction. Sotheby’s had already taken more than £32m at its contemporary art sale on 5 October. Then came the final lot of the evening. No sooner had it gone under the hammer for just over £1m than, to the shock and bafflement of the assembled collectors and art professionals, the work’s own frame started to eat it. Banksy’s Girl With Balloon was pulled downward into what was – with hindsight – a suspiciously bulky frame and emerged sliced into thin strips.

Was it an attack on the art market or a cheap publicity stunt? A Dada masterpiece or just a way for Banksy to raise his prices? Speculation and gossip have fuelled this instant art classic’s fame, yet the cynics have mostly been disproved – so far. Admittedly, in spite of denials by Sotheby’s, the placing of Banksy’s booby-trapped picture at the end of the auction in October looks, to some, too perfect.

“I think it’s almost impossible that Sotheby’s didn’t know about it”, says Miety Heiden of rival auction house Phillips. Yet she doesn’t think Banksy’s stunt has had a massive effect on the artist’s value. Prices for editions of his work have boomed since the Big Shred, but only at the lower end of the market. “I don’t see it at the higher level,” Heiden says. After all, he was already doing rather well. “Obviously it has done nothing to staunch the ever-increasing prices his works are attaining,” exults Banksy’s former agent, the art dealer Steve Lazarides.

What Heiden has seen is people queuing round the block in Hong Kong for a selling exhibition of Banksy that Phillips planned before his sensational iconoclasm. His fame has exploded since and the world is “starting to recognise that he is a great artist”. Lazarides also thinks this has won Banksy a bigger reputation, “especially among the art world hierarchy, who I don’t think have ever given him the credit he deserves”. Banksy has simply carried out “one of the best pieces of performance art ever executed”, he claims. It was “funny, thought provoking and audacious”.

Banksy’s act of auto-destruction created something truly new, a modern masterpiece of mayhem that makes today’s art look equal to forebears such as Marcel Duchamp or Andy Warhol. Just as they proved that a urinal or a soup can may be art, so Banksy has shown that a work of art can become more interesting by being destroyed. He has got something more valuable than money from this. He has proved he is a serious, important artist – and proved it with the funniest prank of the year.