The elusive artist claimed the work as his own by featuring it on his Instagram account yesterday.
The fish depicted on the police box are different in style from the previous six silhouetted designs that Banksy has claimed as his work on successive days over the past week.
One of the designs, featuring a wolf on a satellite dish, was removed by suspected thieves. Another of a stretching cat on an empty billboard was removed by contractors.
Two City of London police officers were initially seen examining Banksy's new work before taking pictures. One officer told the PA Media news agency they had been asked to inspect the artwork after it was picked up on CCTV cameras, and that they were waiting to hear what would be done about it. The governing body of the City of London later said it was working on options to "preserve" the Banksy artwork which has appeared on a police sentry box.
A City of London Corporation spokesperson said in a statement: "We're aware of the works on the City of London police box on Ludgate Hill.
We are currently working through options to preserve the artwork."
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