Households using illegal streaming devices should unplug them immediately to avoid the risk of a fire or electrical accident, campaigners have warned.
A series of tests on nine types of popular illegal streaming devices entering the UK by Electrical Safety First found they all failed to meet safety standards.
The Intellectual Property Office (IPO) estimates that more than one million of the devices have been sold in the UK in the last two years.
In June the EU issued a recall notice for a Chinese-made Android TV box, or Kodi box, amid fears that its poor design left users at risk of electric shock.
The safety warning said the risk of electric shock from the OTT TV Box 4K was "serious" and advised owners to stop using it immediately.
The devices had faulty power supply units which did not comply with Europe's Low Voltage Directive, meaning users were at risk from touching live components.
In July, a raid in London resulted in the seizure of more than 40 illegal streaming devices that provided access to illegal content and lacked UK electrical safety measures.
Steve Curtler, product safety manager at Electrical Safety First, said: "This year consumers thinking about buying an illegal streaming device for Christmas need to know that by plugging them into their TV they could be potentially be putting themselves, their home and their family at risk.
"We urge anyone with one of these devices to unplug it and stop using it immediately. Not only are these devices breaking the law, but they could be putting your loved ones at risk."
Kieron Sharp, director general of intellectual property protection organisation FACT, which jointly investigated the boxes, said: "Alongside the risks of exposing your home network to damaging malware and your children to inappropriate content, it should now be clear that the dangers these illegal devices pose far outweigh any benefit of buying them.
"We believe hundreds of thousands of illicit streaming devices were sold between Black Friday and Christmas last year.
"This year, if you want to safely and reliably enjoy premium sports, TV or films go directly to the official provider."
Consumers looking for a streaming device should buy from a reputable manufacturer and check the box for the manufacturer's brand name or logo and also for a model and batch mark number and a safety CE mark.