One of Banksy's recent artworks appears to be defaced with a tag apparently belonging to a deceased rival.
Create Escape on the wall of the former Reading Prison building, about 60 kilometres west of London, was daubed in red paint with the message 'Team Robbo'.
It is believed to refer to graffiti artist King Robbo who died in 2014. The two were embroiled in a feud after Banksy painted over a King Robbo tag on Regent's Canal in Camden, north London.
Part of Create Escape, which depicts a prisoner scaling the wall using a rope made of paper, was painted over to blot out a typewriter.
Save Reading Gaol, a campaign to turn the former prison into an arts centre, was among the first to discover the overnight addition.
“A sad morning as we learn that the Banksy mural on the side of Reading Gaol has been deliberately damaged with red paint,” it tweeted.
"Awful news to wake up to and to realise that the failure to protect something precious to the town has left it vulnerable to destruction."
The Banksy mural appeared on the prison wall on March 1 and a video posted on Instagram showed it being created.
A spokesman for Reading Borough Council said at the time that the local authority was "thrilled that Banksy appears to have thrown his support behind the council's desire to transform the vacant Reading Gaol into a beacon of arts, heritage and culture with this piece of artwork he has aptly called Create Escape".
Banksy is one of the art world’s most famous names, but he is also largely a mystery.
The street artist, whose thought-provoking works have appeared across the globe, has maintained anonymity despite years of making headlines, thanks to a combination of careful planning and a trusted inner circle.
Here's some more of his work: