Investigators have warned consumers they face potentially fatal risks after 99% of fake Apple chargers failed a basic safety test.
Trading Standards, which commissioned the checks, said counterfeit electrical goods bought online were an "unknown entity".
Of 400 counterfeit chargers, only three were found to have enough insulation to protect against electric shocks.
It comes as Apple has complained of a "flood" of fakes being sold on Amazon.
Apple revealed in October that it was suing a third-party vendor, which it said was putting customers "at risk" by selling power adapters masquerading as those sold by the Californian tech firm.
The Trading Standards tests were performed by safety specialists UL.
They applied a high voltage to the chargers, which were bought online from eight different countries, including the US, China and Australia, to test for sufficient insulation.
Leon Livermore, the chief executive of Chartered Trading Standards Institute, urged shoppers to buy electrical goods only from trusted suppliers.
"It might cost a few pounds more, but counterfeit and second-hand goods are an unknown entity that could cost you your home or even your life, or the life of a loved-one," he said.
A separate operation found that of 3,019 electrical goods bought second hand, 15% were non-compliant.
Officers said the unsafe electrical items, which came from charity shops, antique dealers and second-hand shops, had failings such as counterfeit plugs and basic insulation.
How to spot a dangerous fake charger
Plug pins - Plug the charger into a socket, but don't switch it on or connect to a device. If the charger does not fit easily, the pins may be the wrong size. There should be at least 9.5mm (0.3in) between the edge of the pins and the edge of the charger
Markings - Look for a manufacturers' brand name or logo, model and batch number. Check for the "CE" safety mark, but be aware it can be easily forged
Warnings and instructions - User instructions should include conditions and limitations of use, how to operate the charger safely, basic electric safety guidance and details of safe disposal
Gillian Guy, chief executive of Citizens Advice, said: "Counterfeit electrical goods are likely to be poor quality and in the worst cases unsafe.
"Look out for tell-tale signs of counterfeiting such as mistakes in brand names or logos, and check plugs for safety marks - all genuine electrical items made in the EU should have a CE mark on them."
Consumers were also urged not to overcharge appliances and to never cover devices when charging or use a charger with a cracked case or frayed cable.
There is no suggestion the company involved in the Apple case sold the chargers used in the Trading Standards tests.