A new artwork of swimming fish has appeared on a police box in the City of London, sparking speculation that it might be the latest Banksy.

The elusive street artist has not yet claimed the piece, but if he does it would be his seventh artwork to be unveiled this week in various locations across the capital.

The design on the police box differs from Banksy’s previous silhouette artworks in that the school of fish has been painted with more detail and tone.

It appears the artist has used translucent spray paint to turn the sentry box into a giant fish tank.

Two City of London police officers later arrived to examine the new design before taking pictures from the outside of the police box.

One officer told the PA news agency they were asked to check out the artwork after it was picked up on CCTV cameras and that they are waiting to hear what will be done about it.

Banksy has revealed a new animal artwork each day this week – including a goat, elephants, monkeys, a wolf, pelicans and a cat – by posting a photo of the piece to Instagram at 1pm.

On Saturday, the artist’s sixth piece – a stretching cat on an empty, distressed advertising billboard – was removed from its location in north-west London hours after it was revealed.

Crowds booed as the piece in Cricklewood was dismantled by three men who said they were “hired” by a “contracting company” to take down the billboard for safety reasons.

Hours after Banksy confirmed the design was his, crowds gathered from across London to see the piece before men, who claimed to be contractors, arrived.

A contractor, who only wanted to give his name as Marc, told the PA news agency they were going to take the board down on Monday and replace it, but the removal had been brought forward to Saturday in case someone “rips it down and leaves it unsafe”.

A black board was first used to cover the majority of the cat on the billboard at the request of the police, who wanted to stop people walking in the road in front of traffic.

An officer at the scene told PA that the owner of the billboard has told police he will donate it to an art gallery.

The cat design was the second piece to be removed during the week after a painting of a howling wolf on a satellite dish was taken off the roof of a shop in Peckham, south London, less than an hour after it was unveiled.

It was removed by three men, according to a witness, who told PA that he filmed them, which led to one of the men throwing his phone on a roof.

“It’s a great shame we can’t have nice things and it’s a shame it couldn’t have lasted more than an hour,” he said.

A spokesman for Banksy told PA that the artist is neither connected to nor endorses the theft of the wolf design and that they have “no knowledge as to the dish’s current whereabouts”.

The first piece of graffiti in Banksy’s new animal-themed series, which was announced on Monday, is near Kew Bridge in south-west London and shows a goat with rocks falling down below it, just above where a CCTV camera is pointed.

On Tuesday, the artist added silhouettes of two elephants with their trunks stretched towards each other on the side of a building in the Chelsea area of west London.

This was followed by three monkeys looking as though they were swinging underneath a bridge over Brick Lane, near a vintage clothing shop in the popular east London market street, not far from Shoreditch High Street.

The fifth design, of pelicans pinching fish from a London chip shop sign in Walthamstow, east London, was revealed on Friday.