A mural by street artist Banksy that sparked a local preservation campaign has been defaced.

Spy Booth shows three agents in trench coats and trilby hats eavesdropping on a phone booth.

Banksy Defaced

Conservators are said to be in 'a race against time' to remove the silver and red paint sprayed on Banksy's mural Spy Booth. Banksy works have fetched as much as $1.8 million at auction. (Claire Hayhurst/AP)

It appeared in April on a wall in Cheltenham, western England, home of spy agency GCHQ.

A local group was formed to save the work after the building's owner attempted to have it removed and sold.

Local businessman Hekmat Kaveh had agreed to buy and preserve the mural, but on Friday it was covered in scrawled graffiti letters.

Kaveh said he hoped the work, which was treated with anti-graffiti paint, could be restored.

A local campaigner, Angela De Souza, said conservators were in "a race against time" to save it. 

Banksy works have fetched as much as $1.8 million at auction, and several of his outdoor pieces have recently been stripped from walls and sold.