In a test of 50 unofficial smartphone plugs, 49 were said to fail basic safety checks.
Nearly 50% of the tested chargers failed to pass an electric strength test, posing a strong risk of electric shock.
The counterfeit products were also deemed to have the potential to start fires.
“It is extremely concerning that 49 out of 50 UK chargers we tested failed basic safety checks,” said Martyn Allen, technical director at Electrical Safety First.
“This report shows that anyone purchasing an iPhone charger from an online marketplace or at an independent discount store is taking a serious risk with their safety.
“The majority of chargers we tested had the potential to deliver a lethal electrical shock or cause a fire. We’re urging people to take care when buying a charger and recommend buying directly from trusted retailers only.
“When you buy a fake, at best you could damage your phone but at worst you could be putting your life, your family and your home at risk.”
Electrical Safety First warns those purchasing iPhone chargers to look out for a high quality finish to indicate good build quality, and for a weight of at least 40g.