Staff on London’s public transport network have been warned that blades are being concealed inside posters promoting conspiracy theories about Covid-19 and vaccinations.

Transport for London (TfL) said there had been a number of reports of razor blades being attached to the back of the posters intended to harm anyone taking them down, and that at least one person was harmed in an incident outside its network.

Trade union representatives for transport workers, who likened the tactic to that used in the past by fascists from the National Front, have raised the issue on a safety forum for staff and management and are to provide a warning to members.

A bulletin sent by TfL said “propaganda posters” questioning the existence of Covid-19, spreading untruths about vaccinations or carrying other messages, had been placed in locations such as doors, lamp-posts and walls.

The bulletin included two images, one of a poster saying “Masks don’t work”, and another showing its reverse spattered with blood and being held by someone carrying a bloodied tissue.

The images appear to be from an incident in July in which a woman in Wales was cut by a razor blade found glued to the back of a poster spreading misinformation about the virus. Police launched an investigation into the incident, in which the woman received a deep cut after taking down the poster in Cardiff.

Similar bulletins to the one sent by TfL were issued last month to health workers who were told about the misinformation posters. They were reportedly told that anti-vaccination posters, in some cases containing blades, had been put up in areas near to or on vaccination sites.

Mick Lynch, the general secretary of the RMT union, 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.”

A TfL 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.”

The mayor of London, Sadiq Khan, on Tuesday criticised what he described as a “selfish minority” who refused to comply with mask-wearing on buses and the TfL network, where it continues to be mandatory for people to wear face coverings.

The mayor, who is also chair of TfL, said enforcement officers would not hesitate to order passengers off TfL services who defy requests to don a face covering.

Activists have also filmed themselves pulling down public health information posters on the London Underground, where stickers spreading conspiracy theories about the pandemic have become increasingly commonplace.

In a stunt last month a man wearing a high-visibility vest and posing as a “Covid marshal” was filmed walking through a carriage with a loudspeaker telling passengers their “papers” were to be checked to ensure they had been vaccinated.