LONDON, June 10 -- The charger arrived in an Apple box, bore the Apple logo, and cost £11.99 on eBay. When researchers at the consumer watchdog Which? cracked it open, they found modelling clay.

Not a circuit board defect. Not a missing component. Modelling clay - the kind used in primary school art classes - packed inside a counterfeit USB-C power adapter to give it enough heft to feel genuine in the hand. Ten seconds into a standard electrical strength test, arcing sounds confirmed what the clay already suggested: current was jumping between parts of the circuit in ways that could cause the device to catch fire, explode, or deliver a lethal shock to whoever was holding it. One customer had already discovered this without a laborat...