A London transport union has warned workers after posters were discovered with razor blades on the back.
The RMT union warned workers that some anti-mask posters were found to have blades stuck on the back, designed to injure staff cleaning stations.
A bulletin issued by Transport for London (TFL) warned staff of a poster outside of the network which had injured at least one person.
RMT secretary Mick Lynch said that they had reported the incidents to TFL and had been helping with investigations.
"We would expect the police and the courts to take the hardest possible line," he said.
Transport for London that all graffiti posters or bills on their network were removed by staff, reported the Guardian, and they were taking the issue seriously.
"We act urgently to remove unauthorised materials from our network, such as posters, including those relating to the pandemic," said a spokesperson for the network.
"The safety of our customers and colleagues is paramount and we have issued guidance to our staff on how to remove them safely after reports of instances outside of our network of razor blades being attached to the back of such materials."
In July, 22-year-old Layla Stokes in Cardiff was reportedly injured by a similar poster at a rail crossing in the Welsh capital.
South Wales Police told the BBC they were investigating CCTV to identify those responsible.
Mayor of London Sadiq Khan recently criticised the "selfish minority" who oppose mask-wearing on public transport.
Face coverings continue to be a requirement on TFL services, though mask-wearing rules on other UK public transport have been eased.
However, Khan said he was "not in favour" of vaccine passports being needed to travel.
"It simply would be almost impossible for us to get it working without having massive, massive queues leading to big problems," he told LBC radio in July.
"We've got to encourage vaccine take-up, but I think it's not realistic to expect us to have vaccine passports within London."