Thousands of these devices have been confiscated by officers at U.K. ports following tests that uncovered safety issues in plugs, cabling, chargers, batteries, and cutoff switches, the Guardian reports. Some of the devices are at risk of overheating, which can result in explosions or fires.

British retailer Halfords issued a recall this past weekend of its own brand, the Air Runner Balance Board. Marketed as a children’s “ride on toy,” it was being sold with plugs and chargers that don’t comply with the country’s standards. Costco issued a safety notice about the hoverboards, sold at U.K. stores, in November cautioning consumers about an electrocution risk.

Lord Toby Harris, chair of the National Trading Standards, told the Guardian that the agency’s teams have noticed a significant increase in the amount of dangerous hoverboards entering the country recently, and they are “working round the clock to prevent dangerous items from entering the supply chain.” The products are entering the country to be sold by domestic retailers. He advises anyone planning on purchasing a hoverboard to read the agency’s product safety list.

Leon Livermore, chief executive of the Chartered Trading Standards Institute, adds to Harris’ warning:

Criminals and irresponsible manufacturers will often exploit high demand and attempt to flood the market with cheap and dangerous products. Some products that are made abroad, principally for the overseas market, are not fitted with the correct plug and fuse for use in the UK.