LONDON >> Art prankster Banksy has struck again.

A work by the elusive street artist self-destructed in front of startled auction-goers on Friday, moments after being sold for 1.04 million pounds ($1.4 million). In an Instagram post today, Banksy claimed the dramatic artistic payoff had been years in the making.

The spray-painted canvas “Girl With Balloon” went under the hammer at Sotheby’s in London, fetching more than three times its pre-sale estimate and equaling a record price for the artist.

Then, as an alarm sounded, it ran through a shredder embedded in the frame, leaving half the canvas hanging from the bottom in strips.

A post on Banksy’s official Instagram account showed the moment — and the shocked reaction of those in the room — with the words “Going, going, gone…”

A video was later posted on the account, stating: “A few years ago I secretly built a shredder into a painting in case it was ever put up for auction.” The video showed images of a shredder being implanted into a picture frame along with footage of Friday’s auction finale.

Banksy’s spokeswoman, Jo Brooks, confirmed that the post was genuine.

Sotheby’s — which had noted before the sale that the work’s ornate gilded frame was “an integral element of the artwork chosen by Banksy himself” — appeared as shocked as anyone else.

“It appears we just got Banksy-ed,” said Alex Branczik, head of contemporary European art at the auction house.

The auction house said it was “in discussion about next steps” with the buyer, whose identity was not disclosed. Some art-market watchers say the work could be worth even more in its shredded state.

“We have not experienced this situation in the past . where a painting spontaneously shredded, upon achieving a record for the artist,” Branczik said. “We are busily figuring out what this means in an auction context.”

Geneva-based artist Pierre Koukjian, who was at the auction, said the buyer was “very lucky” to own a now-historic piece. He called Banksy’s prank “a turning point in the history of contemporary and conceptual art.”

Koukjian, who has met Banksy, said he is sure he caught a glimpse of the artist in the saleroom amid the confusion of the moment.

“What he did is really shocking, in a good way,” Koukjian said. “I think it will be historic and people will talk for a long time about it.”

Brooks would not say whether the artist had been at the auction. She said Sotheby’s had been “100 percent” unaware of the planned stunt.

Banksy is not the first artist to deconstruct his own work. In the years after World War II, German-born artist Gustav Metzger pioneered “auto-destructive art,” creating paintings using acid that ate away the fabric beneath.

Banksy, who has never disclosed his full identity, began his career spray-painting buildings in Bristol, England, and has become one of the world’s best-known artists. His mischievous and often satirical images include two policemen kissing, armed riot police with yellow smiley faces and a chimpanzee with a sign bearing the words “Laugh now, but one day I’ll be in charge.”

He also has a penchant for elaborate pranks.

In 2005, he hung an image of a spear-toting ancient human pushing a shopping cart in the British Museum, where it remained for several days before being discovered. The next year he smuggled a life-sized figure of a Guantanamo Bay detainee into Disneyland, and in 2015 he erected a full-scale dystopian theme park — “Dismaland” — by the British seaside.

“Girl With Balloon,” which depicts a small child reaching up toward a heart-shaped red balloon, was originally stenciled on a wall in east London and has been endlessly reproduced, becoming one of Banksy’s best-known images.