EARLY ONE morning in February, a team of specialists pulled up next to a building in Nottingham and began drilling. They worked quickly but carefully, excavating a small section of the wall on which “Hula-hoop Girl” had been painted by Banksy, a prominent street artist. John Brandler, a gallerist, said that he had purchased the artwork—which depicts a youngster playing with a bicycle tyre—from the building’s landlord on behalf of a client. Local residents were horrified. The mural had spread cheer during the lockdowns in Britain, when galleries and other cultural institutions were shut.

It was not the first time Banksy’s artwork has been carried off; Mr Brandler said he had used the removal services of this “very specialised company” before. So many of Banksy’s pieces, in particular, have been taken and sold into private collections that they have been brought together to form an exhibition. “The Art of Banksy”, the world’s largest display of his work, opened in London in May as a paid-for, ticketed exhibition after stints in Auckland, Melbourne, Miami, Sydney, Tel Aviv and Toronto.

The show invites art lovers to stump up cash to view privately held works intended to be publicly accessible and free. Banksy has criticised the practice, saying that he will not authenticate artworks removed from their original site. But rather than shy away from controversy, the organisers are embracing it: in their words, the exhibition is “completely non-consensual”, meaning it has not been authorised by the artist.

Street art is an unusually precarious art form. Ben Eine, a celebrated British artist, has been arrested numerous times while spraying his colourful wall art, which commonly spells out words. Painters rarely seek permission to turn a building into a canvas, and so while the artwork belongs to them, the backdrop does not. When Banksy painted “Achoo!!” (pictured), a mural of an old woman sneezing, on the side of a house in Bristol in December, it raised the value of the property. (It has since been removed and will be sold at auction.)

If they want to protect their pieces from unwanted buyers, artists should ask for permission from the building owner and sign a contract, says Joshua Schuermann of Briffa, a legal firm specialising in intellectual-property law. Mr Eine agrees: “It’s the nature of the business and the game.”

Another, different problem is posed by many street artists’ decision to remain anonymous, Mr Schuermann says. By using secret identities, artists shield themselves from prosecution for vandalism but potentially lose out on the chance to protect their works from copyright theft. Unable to satisfy the legal requirements to prove that the work being exploited is theirs, many can only watch on with disillusionment as it is co-opted by people with their own commercial interests.

But the legal protections are improving as more people come to appreciate street art’s value. In a case heard in America last year, a judge found that famous graffiti in New York was protected from removal because it had “significant artistic merit and cultural importance”. The claimants, 21 graffiti artists who had their work cleaned off, received $6.75m in compensation from the property developer who ordered the whitewashing. Copyright lawsuits are also increasingly common. In 2014 Ahol, an artist from Florida, sued American Eagle, a clothing company, for “blatant, unlawful and pervasive infringement” of his work after they used his signature eyeball design (the case was settled out of court). Other companies, such as McDonald’s and Audi, have been criticised for featuring street art in their advertising without consent.

For street artists, the commercialisation of their artform rankles because it is meant to be an unpretentious, democratised mode. Mr Eine started his career vandalising the streets of London, but as he grew older he sought to produce work that would enhance its environment. “I wanted to make art that children would point at and old ladies would like,” he says. Parents tell him that his alphabet letters, painted on the shutters of a row of shops, help their children to practise their A to Z as they go about their daily lives. That would not be possible if they were sequestered away in a private home.