Some are selling powerful Cali weed for as much as £40 a gram.
Most operate behind blacked-out windows. Cafes have already been tracked down to Shoreditch and Limehouse, east London, while others are known in Birmingham, Manchester, Leeds and Newcastle, although the venues are said to be temporary.
Some strains for sale contained psychoactive tetrahydrocannabinol.
Ian Hamilton, addiction lecturer at the University of York, said: “You could get a range of possible effects – you can feel more anxious and on edge.
“That’s the least that could happen, and the worst is several symptoms similar to psychosis.”
Meanwhile Home Secretary Sajid Javid has been urged by a group of cross-party MPs and peers to sanction drug rooms, allowing addicts a safer place to consume their supply, with sterilised equipment, medical help and advice on hand.
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