A Europe-wide investigation by consumer group Which? found 66% of products – including carbon monoxide alarms, baby toys and travel adaptors – bought from third-party sellers on the most popular sites failed EU safety requirements.

Which? experts tested 250 products on sites including Amazon, eBay, AliExpress and Wish over the last year and 165 of them failed official safety tests.

Many were found to contain harmful substances, inferior design or unclear information.

Among the findings, researchers discovered smoke and CO alarms that couldn’t detect smoke or CO, Christmas lights that could give you an electric shock and USB chargers and travel adaptors that could cause a fire.

Alarmingly, every single product tested in the teeth-whitening, CO alarms, balloons, smoke alarms and helmet categories were deemed safety failures.

The shocking findings demonstrate why greater regulation for online marketplaces is needed, Which? said.

Online marketplaces are not currently responsible for the safety of the products sold through their sites.

Neena Bhati, head of campaigns at Which?, said: “Online marketplaces have quickly become a popular way for people to shop online and yet Which? testing has repeatedly exposed how large numbers of dangerous products are sold on these sites everyday.

“These platforms have failed to get a grip on this despite years of warnings, and so the government must now step in to make online marketplaces responsible for ensuring the safety of the products sold on their sites.”

What the sites say

An Amazon spokesperson said: “We require all products offered in our store to comply with applicable laws and regulations and have developed industry-leading tools to prevent unsafe or non-compliant products from being listed in our stores.

“Sellers are responsible for meeting Amazon’s high bar for product quality and we may remove and take legal action against those who do not.”

An AliExpress spokesperson said: “We have strict platform rules that require all third-party sellers to comply with all applicable local laws and regulations.

“AliExpress has strong processes and technology in place to help prevent the listing of any products that violate our policies, such as proactively screening for and taking down any non-compliant listings.”

An Ebay spokesperson said: “These listings have been removed and we have taken enforcement action against the sellers.

“We have no tolerance for unsafe, banned, recalled, or illegal products on our marketplaces, and it is required in our User Agreement that all sellers comply with the law.”

A Wish spokesperson said: “All of the merchants on our platform are required to adhere to local laws and safety standards wherever their goods are sold. In the rare instance where a product falls below those standards and sufficient evidence is provided, we take the appropriate action to remove the items as swiftly as possible. In some cases, we also apply sanctions to merchants who have intentionally circumvented our policies and safety standards.