Anti-vaccine posters put up around Tube and train stations in London have been found with razor blades on the back.
Anyone attempting to take them down would be injured, according to the Rail, Maritime and Transport union (RMT).
The RMT revealed its findings last week and said staff had now been warned they could be at risk of injury if they take the posters down.
The posters read: “Masks Don’t Work.”
The message is likely to be linked to the London mayor, Sadiq Khan, and his decision to make face masks mandatory when travelling on London services back in July.
His announcement followed prime minister Boris Johnson’s unlocking of the entire country two months ago, where he also lifted the law surrounding obligatory mask-wearing in enclosed public places.
Upon discovering the dangerous posters, RMT general secretary Mick Lynch said: “Any anti-vax conspiracy theorists 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.”
Transport for London (TfL) maintained it was not aware of any incidents linked to razor blades, and there have been “very few instances of unauthorised adverts and materials” linked to the pandemic.
But the network promised to “act urgently” to remove such “unauthorised materials”.
The spokesperson said: “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 attacked 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.”
Anti-vaxxers have repeatedly hit headlines in recent weeks after their attempts to storm old BBC studios and the Google offices in central London. The protesters claimed the “media was the virus” for covering the pandemic.
Demonstrators even attempted to break into the Medicines and Healthcare products Regulatory Agency (MHRA) headquarters last week, and have carried out several protests against jabs for those under-18 and vaccine passports in recent months.