Art lovers are in a "race against time" to save a Banksy mural which targets the issue of Government surveillance, after the piece was vandalised.
Letters in silver and dark red paint were sprayed on the piece, called Spy Booth, in Cheltenham, Gloucestershire, overnight.
Spy Booth shows three 1950s-style agents, wearing brown trench coats and trilby hats, using devices to tap into conversations at a telephone box.
Local business leaders joined forces to protect the artwork from being sold to an American investor, who wished to remove it from the wall.
Millionaire property developer Hekmat Kaveh, who lives in the town, stepped in to buy the piece, which had a £1 million asking price.
Graffiti appeared over Spy Booth, which is painted on the side of a house situated a few miles from GCHQ, where the UK's surveillance network is based.
The artwork had previously been protected with anti-graffiti paint, though the dark red and silver letters could still seep through it.
Residents armed with toothbrushes and cloths have attempted to remove the graffiti but experts are set to complete the task tomorrow.
Mr Kaveh said a clear perspex would then be placed over the piece, which he intends to keep at its current location.
"I am absolutely appalled at the graffiti," he said. "But it has only given me more determination to make sure that we, Save The Banksy, do actually save it - not just from movement but from vandals.
"I have put up money for the purchase. I can't divulge how much - that is the arrangement between the parties.
"Everybody knows that the asking price was £1 million based on previous Banksy amounts but I can't say if it was above or below that."
Mr Kaveh said he expected to gain ownership of Spy Booth "within days", following a lengthy legal process.
The piece appeared overnight in April, with Banksy later officially confirming it on a link titled Q&A on his website.
Angela De Souza, of campaign group Save The Banksy, described the process to restore the artwork as a "race against time".
"Everybody is just really, really upset," Mrs De Souza said. "We were worried about securing the piece but we didn't expect an attack so soon.
"The Banksy is protected by anti graffiti paint but we are in a race against time because the paint could seep through the layer of protection and ruin the artwork."
Shocked locals visited the site to express their anger at the damage.
Builder Martin Burnett, 48, said he was "furious" the Banksy had not been removed from the building.
"It should have been taken off the wall and put in a museum for everyone to view it there," he said.
"It should have been taken away instead of being left here for this idiot to come and do this."
Gloucestershire Police were called to the Banksy at 8am today, with officers maintaining a presence in the area since then.
Detective Inspector Angela Middlewood said: "We have received a report regarding the Banksy artwork in Cheltenham. Officers are at the scene and enquiries are being carried out.
"I would like to take this opportunity to ask anyone who was in the area last night or in the early hours of this morning and saw something out of the ordinary, to contact us on 101."