A piece of Banksy street art painted on the side of a Poundland store has sold to a US artist for more than £500,000 – but the buyer has vowed to whitewash it in protest.

Ron English says he plans to paint over the north London mural because street art “shouldn’t be bought and sold”.

Slave Labour depicts a young child on his knees at a sewing machine, producing a string of Union Jack bunting. Banksy painted it on the side of the bargain store in Wood Green in 2012.

Many believe the painting was a protest against sweatshop-manufactured souvenirs for the London Olympics. Authorities removed it from the wall in February 2013, to the anger of residents.

“My idea for this painting is to whitewash it for my good pal Banksy, I only wish I could’ve spent more money for it,” he said.

“This is a blow for street art. It shouldn’t be bought and sold.”

Last month, a Banksy piece sold for more than £1m partially “self-destructed” after the hammer came down.

Girl With Balloon, one of the anonymous artist’s most famous works, passed through a secret shredder installed in the frame.

The buyer later confirmed they were going through with the sale.

Posting on Instagram about the stunt, Banksy said: “Some people think it didn’t really shred. It did. Some people think the auction house were in on it, they weren’t.”