Dangerous e-bike chargers were found on sale on Amazon and eBay, according to consumer safety charity Electrical Safety First (ESF).
The charity said online shopping sites Wish.com and AliExpress were also selling the chargers, which can cause risk of fire and electric shock.
It said all chargers investigated failed to meet UK safety standards and they have since been removed from sale.
All four sites said they had safety procedures in place.
The chargers are used to power the lithium-ion batteries which are used by e-bikes.
Most were predominantly advertised as accessories for e-bikes, but the listings for some suggested that they can be used to charge e-scooters and hoverboards.
Martyn Allen, ESF technical director, said that the chargers that were found "should never have been available for sale to UK shoppers".
"The lack of vital regulation needed to ensure online marketplaces are responsible for the safety of goods sold via their platforms is contributing significantly to dangerous products entering people's homes," he said.
The London Fire Brigade has issued a warning after an increase in fires caused by e-bikes – although there is no claim faulty chargers are to blame.
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One blaze in Southwark, which was linked to an e-bike, resulted in 70 firefighters having to attend the scene.
'Risk of serious fire'
The charity looked at the mains plugs in the charging devices and said that all were so visually substandard that no test purchasing was necessary.
ESF said all of the chargers failed to meet the UK plug standard, with most appearing not to have a fuse – a situation which has the potential to cause a serious fire.
The investigation also found the plugs advertised appeared to be smaller in size than the UK standard, increasing the risk of electric shock if a consumer came into contact with live plug pins.
It said the danger of incorrect plugs means they have not been "adequately tested" which means other things can be wrong with them.
It found nearly 60 listings for the chargers across the four marketplaces, with the most appearing on eBay, which had 21.
Each of Amazon and Wish.com had 13, while there were 12 listed on AliExpress.
A spokesperson for eBay said the company takes consumer safety seriously and, along with using its security teams, it works closely with stakeholders to keep unsafe products off the site.
Spokespeople for AliExpress and Wish.com said merchants must comply with its policies, while Wish said it has asked the merchants selling the products for further compliance documentation.
Amazon said it has measures in place to prevent suspicious products from being listed and monitors products sold for safety concerns.