Picture: Banksy/Instagram

London - Prankster artist Banksy has revealed how he carried out his biggest stunt yet.

The art world was caught by surprise when his most famous work shredded itself seconds after selling for £1million at auction.

But it appears the graffiti artist had been keeping the trick up his sleeve for years.

Banksy posted a video on Instagram at the weekend showing how he built a shredder into one of his paintings.

The clip starts with a caption saying: ‘A few years ago, I secretly built a shredder into a painting in case it was ever put up for auction.’

It then shows a hooded figure placing the shredder into a wooden frame, before showing images of the Sotheby’s auction of Balloon Girl – and the spray painting’s self-destruction.

The clip is captioned ‘The urge to destroy is also a creative urge’ – a quote from 19th-century anarchist Mikhail Bakunin which Banksy attributes to Pablo Picasso.

It remains unclear how the shredder was activated at the precise moment after the hammer went down in London.

But – no doubt to the delight of Banksy – the episode has led to an outbreak of rumours over the culprits, with some saying the artist was in the auction room himself on Friday night and others pointing the finger at Sotheby’s themselves as taking part in an elaborate joke.

Another film of the auction appears to have captured the artist himself videoing the artwork being shredded.

It shows a man looking very much like the former public schoolboy previously unmasked as Robin Gunningham seemingly poised and ready for the moment which captured most others in the room off guard.

Speculation has also focused on exactly who had purchased the 2006 artwork after it went to a telephone bidder for a total of £1.04million (around R19 million) – equalling a record for a Banksy.

Soon afterwards the artist posted a video of the scene with the caption ‘Going, going, gone’, leading some to say Banksy himself was the buyer, in collaboration with Sotheby’s.

Others said the stunt had increased the value of the work, while there were also theories over whether the buyer would be expected to go through with the purchase.

A spokesman for Sotheby’s said: ‘We have talked with the successful purchaser who was surprised by the story. We are in discussion about next steps.’

The Art Newspaper’s correspondent Anny Shaw, who was in the auction room, said that while the shredding may have been an ‘inside job’ it certainly looked like a surprise.

She added: ‘An alarm was activated just before the moment of self-destruction happened and that led to the speculation that there was someone in the room, possibly even Banksy.

‘I subsequently heard there had been reports of a skirmish between security staff and a man dressed in black and sunglasses exiting the building. You couldn’t make this up.’

Banksy – whose previous stunts include Dismaland, the ‘family theme park unsuitable for children’ in Weston-super-Mare, Somerset – has refused to reveal his identity.

Mr Gunningham, 45, from Bristol, was first ‘unmasked’ as the real Banksy in 2008, but has since made no moves to deny or confirm the report.