Children’s car seats that are made from flimsy material that breach EU safety rules, and have been shown to offer no protection at all for child passengers, can still be bought from some major online retailers, Which? has revealed.

The consumer watchdog said the “killer” seats gave children almost no protection from the force of crashes but were described as suitable for newborns and children up to five years of age in the online listings of some popular sites.

The cloth seats lack any form of recognisable support or protection but researchers for Which? found them for sale on eBay, Amazon and AliExpress for as little as £8.

It said online martketplaces should have removed the obviously inadequate seats from sale as soon as they were listed to stop British consumers buying them in the mistaken belief they were safe to use.

In 2014 Surrey trading standards and manufacturer Britax showed that the seats could be potentially lethal after carrying out tests that showed the material in the restraint disintegrating completely in a 30mph crash test.

The devices were described as “killer car seats” after a crash test dummy of a three-year-old child was thrown through the windscreen of the car when the straps snapped.

Which? found evidence they had repeatedly reappeared for sale online despite the fact that only EU-approved child seats were legal to be sold in the UK.

The watchdog, which recently claimed that the UK’s consumer enforcement system is on the brink of collapse, is calling for a reformed inspection system to monitor sellers trying to flout safety regulations online.

Alex Neill, head of Which? home products, said: “More needs to be done by big businesses and government to protect consumers from dangerous products.”

Ebay said it had asked the sellers involved to contact the buyers to organise a return and to pay for the return shipping.

A spokeswoman from the company said: “Our specialist teams work with regulators and trading standards to ensure our block filters stay up to date, using sophisticated software that monitors billions of listings a day to remove any prohibited items.”

Amazon removed the items from sale and in a statement said: “All sellers must follow our selling guidelines and those who don’t will be subject to action including potential removal of their account. The products in question are no longer available.”

Retailer AliExpress said: “AliExpress considers the safety of all our customers, especially children, to be of paramount importance. We will continue to take action against sellers who violate our terms of use.”