Some of the products were featured in fake reviews and dodgy ads, leaving people at risk of fires or electric shocks.

Ten mini plug-in heaters - some as cheap as £12 - were tested by Which? and all of them failed safety tests. Four of them - three on eBay and one on Amazon - were a fire risk.

All of the models tested did not meet the Electrical Equipment Safety Regulations.

Rocio Concha, Which? director of policy and advocacy, said the products were illegal, could cause serious harm, and show that online platforms must do more to protect consumers.

She added: "The government must urgently legislate to strengthen product safety, including giving online marketplaces greater legal responsibility for unsafe products sold on their sites so that consumers are far better protected.

"Self-regulation of online platforms is simply not working. These platforms have put in place inadequate measures which leave consumers at risk. The government must give these platforms greater responsibility to protect people online."

Among the problematic heaters was the Plug-In Wall Heater 500W sold by a third-party seller on eBay.

Which? said the plastic surround holding the heating element in place melted and the heating element slipped down to create another melted area, which was against the outer grill, meaning that touching it could result in shock or burns.

Several versions of this heater sold on Amazon, eBay and Wish all failed, and Trading Standards also issued a recall notice for another model sold on eBay that appeared to be identical.

An electric portable plug-in heater, sold on Amazon, had counterfeit fuses that had been used in the plug for the heater. They posed a risk of electric shock, fire or even an explosion, Which? said.

Another plug-in wall heater, sold on eBay, posed a risk of fire or electric shock, with internal wires soldered directly onto a circuit board, while another circuit board was just loose within the product.

An Amazon spokesperson said: "Safety is a top priority at Amazon and we require all products offered in our store to comply with applicable laws and regulations.

"Two products were removed in December, including one in relation to safety concerns, and we have proactively removed similar items and introduced measures to prevent new products going on sale.

"We have removed the remaining two products while we investigate. If customers have concerns about an item they've purchased, we encourage them to contact us directly so we can investigate and take appropriate action."

An eBay spokesperson said: "We take the safety of our users very seriously. We had already removed three of the five listings that Which? flagged to us. One of the two remaining listings is selling a different product and the other one has already ended.

"We use block filter algorithms aimed at preventing unsafe products from being listed. These filters blocked 4.8 million listings in 2022 and are updated on a regular basis.

"On the rare occasion that an unsafe product does make it on to site, we swiftly remove it and provide product safety education to the sellers to prevent relisting."

Wish said: "Product safety is a top priority for Wish. The listing highlighted by Which? has been removed from our European platform, and we are monitoring for any identical or similar listings. We will take further action as appropriate."

Google was asked for comment but had not provided one.