Snorting Copper was missing for a decade after being vandalised and boarded up – now it has been uncovered in east London

A Banksy painting that appeared on a public toilet block in east London, only to disappear after it was vandalised, spray-jetted by the local council and then painted over, has been rediscovered over a decade later.

Known as the Snorting Copper and considered an exemplary image by the elusive graffiti artist, it shows a uniformed policeman on his hands and knees snorting a line of cocaine.

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Although the painting has been valued for insurance purposes at £1.25m, Jonathan Ellis and David Kyte, who uncovered the painting after they bought the disused site, say they have no intention of selling it. Instead, they are restoring it and will return it to its original site in Shoreditch, Hackney, so that the general public can see it. The unveiling will take place on 5 October.

“It’s an amazing piece,” Ellis told the Guardian. “We’ve had offers to sell it. But we want to put it back. We think that’s the right thing to do for the public to enjoy it. I’m proud to be able to do something like that.”

John Brandler, a specialist dealer in Banksy artworks, said the painting was a famous image, reproduced in several books, “but no one knew where it was”.

Ellis said the painting had been vandalised before “Hackney council spray-jetted it and someone else painted it white”. It was then boarded over.

In a complex operation, the entire wall has been cut out and sent for restoration in Carlisle, Cumbria.

After visiting the restorer’s studio, Brandler said: “I was blown away because I’d only seen [the painting] in books … It shows how a non-state approved artist can make some very punchy political points. Most cartoonists, when you look at their work, it’s very topical for a day or a week … This is a political comment and 10 or 20 years later, it’ll still be of interest.”

He is aware of two other versions, but believes that this is now the only surviving original. Asked how he can be sure it is an authentic Banksy, he said: “It’s recorded in two authorised books on Banksy and the location is recorded.” The fact that it had been painted and boarded over actually preserved it, he says. “It’s ironic really,” he added.

Other Banksy works have been vandalised or destroyed. In 2014 a Banksy artwork in Clacton-on-Sea depicting pigeons holding anti-immigration banners was removed by the local council after someone complained it was racist. “One person complains they found it offensive and the council water-jets it off the wall. If the council hadn’t, they could have been prosecuted. If one person complains, it has to be removed,” Brandler said.

The developers purchased the single-storey toilet block and next-door building to convert them into flats and offices. The public convenience had not been used for years and the previous owner had bought it from Hackney council, Ellis said.

Having been involved with another nearby development, he had heard from locals about the Banksy. “They said, ‘you know there’s a Banksy up the road painted over by Hackney.’ The day we completed and got the keys, I went down there. We carefully took the plywood off the wall,” he said. Seeing the painting’s whitewashed surface, they arranged to cut the entire section of wall and have it transported to Chris Bull of the Fine Art Restoration Company in Carlisle.

Bull has painstakingly removed a heavy layer of industrial white paint as well as graffiti, and done other restoration work on the piece. He said: “We used different solvents to soften them and scalpels to peel them back.”

Asked about security, Ellis said the artwork would be bolted to a concrete floor. It weighs 2.5 tonnes and will be protected by reinforced glass, alarms and CCTV. In its steel frame, it is about 2.5 metres wide by 1.5 metres high.

Ellis said: “We’re putting it back in exactly the same location it was painted on the toiletblock, but we’ve put it back on the inside of the glass, so it can’t get vandalised or damaged. We’re lighting it up 24 hours a day so you can see it.”

Banksy, known for his stencil-based images, has maintained his anonymity, despite repeated attempts to unmask him. Describing himself as a “quality vandal”, he has made his name by poking fun at authority figures through artworks in public places. Stencil enables him to work at speed, quickly disappearing into the night.

In 2005, he smuggled his own works into major museums including Tate Britain in London and the Metropolitan Museum in New York. In 2015, he opened Dismaland in Weston-super-Mare, a temporary amusement park and conceptual art show.

In July, his Girl with Balloon – in which a child watches her heart-shaped balloon drift away – was voted Britain’s best-loved work of art, ahead of masterpieces such as Constable’s painting The Hay Wain.

• On the Record, an exhibition of Banksy record cover designs, opens at the Brandler Galleries in Brentwood, Essex, on 19 August.