Electrical Safety First also tested 14 EU chargers as part of this research. All 14 failed all of the safety tests in every respect.
"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.
"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."
Apple has introduced wireless charging on the latest versions of its smartphone, the premium iPhone X and the iPhone 8 and iPhone 8 Plus models.
It has not brought out its own branded charging pad yet, although several third-party chargers exist.