image source, Layla Stokesimage captionA woman was injured when trying to remove a similar poster from a crossing in Cardiff
London transport staff have been warned after anti-mask posters with razor blades on the back were found, a union has said.
The Rail, Maritime and Transport (RMT) union said it had raised the issue of "Masks Don't Work" posters with Transport for London (TfL).
TfL said it was not aware of any incidents involving blades but had given advice to its workers.
A similar poster was found in Cardiff where a woman was injured removing it.
The RMT called for the courts to take the "hardest possible line" against those found responsible.
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The RMT has formally raised the issue with TfL and said warning bulletins highlighting the danger of injury had been issued to staff.
General secretary Mick Lynch said: "Any anti-vax conspiracy theorist resorting to this disgusting practice of lacing their propaganda with razor blades needs to know that they will face criminal prosecution and the highest possible sentences.
"As far as RMT is concerned they should be locked up for a long time. We would expect the police and the courts to take the hardest possible line."
image source, RMTimage captionTfL has sent guidance to staff after the incident in Cardiff
TfL said it was not aware of any incidents involving razor blades taking place on its network.
A spokesperson said: "We act urgently to remove unauthorised materials from our network, such as posters, including those relating to the pandemic.
"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.
"None have been detected on our network and there have been very few instances of unauthorised adverts and materials relating to the pandemic."