A Banksy mural targeting the issue of government surveillance has been vandalised with silver and red spray paint.

Residents saw a group of men packing away a large white tarpaulin on Fairview Road, before driving off in a maintenance van.

Spy Booth showed three 1950s-style agents, wearing brown trench coats and trilby hats, using devices to tap into conversations at a telephone box.

It appeared overnight in April on a street in Cheltenham, Gloucestershire, just a few miles from GCHQ, where the UK's surveillance network is based.

The celebrated Bristol artist later officially confirmed he created the piece.

Since then, the Cheltenham business community has been attempting to raise funds to keep the piece, which had been sprayed with anti graffiti paint, in the town.

Angela De Souza, of Save The Banksy, a group of Cheltenham business owners, said a local millionaire had been in the process of buying the piece.

But on Friday morning, graffiti covering the artwork appeared, leaving locals in a "race against time" to save it.

De Souza said: "Everybody is just really, really upset. As far as I know, contracts were being written and examined yesterday. We were all so delighted and then this morning we heard of the graffiti; it is devastating.

"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."

The vandalised Banksy artwork. Photograph: Newsteam/SWNS / Newsteam

Shocked locals arrived at the site to examine the damage.

Builder Martin Burnett, 48, said he was furious that the Banksy had not been removed from the building.

"I believe what should have happened is 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 left here for this idiot to come and do this. The council should have put a copy of the Banksy here, covered it in perspex and preserved the original."

Gloucestershire police appealed for anyone with information about the graffiti to contact them.

DI 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."