Cleaners were unaware that the graffiti on a London underground train was by world-renowned artist Banksy when they removed it, the BBC learned.
The piece, If You Don’t Mask, You Don’t Get, was painted inside a Circle Line service cart.
But by the time he released the work on his Instagram account, it had been erased by the cleaning crews at Transport for London (TfL).
A TfL source said: “It was treated like any other graffiti on the network. ”
“The job of the cleaners is to make sure the network is clean, especially given the current climate,” they said.
A video posted online shows a man – presumed to be Banksy – disguised as a cleaner and armed with stencils.
The waterfall, which was revealed on Tuesday, is believed to have been designed to encourage the use of face masks.
By Tom Edwards, BBC London Transport correspondent
It was a stain on a cleaning cloth long before the artist revealed on social networks that he had done it.
In today’s climate, it may be reassuring to see that the tube cleaners have done their job quickly and efficiently and cleaned up the job so quickly.
Graffiti is seen – certainly in the transportation world and by many commuters – as something that contributes to a threatening and unwelcoming atmosphere.
Sure, some would argue that it should have been preserved or protected as an art, but it’s a bit academic.
You get the impression that Banksy, who already destroyed his art on purpose, knew exactly what would happen to his job by putting it in a car.
It may have been part of the plan.
An official statement said that the art was removed “a few days ago” in accordance with the “strict anti-graffiti policy” of the London Underground.
All users of public transport in London must wear a face cover, and TfL said it appreciated “the feeling of encouraging people” to do so.
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“We would like to offer Banksy the opportunity to rewrite their message for our customers in an appropriate location,” he added.