New research has revealed that a staggering 66% of the 250 products bought from online marketplaces have failed safety tests.
We've spent the past year working in collaboration with five other European consumers' associations to test products sold through online marketplaces such as AliExpress, Amazon, eBay and Wish to see how safe they are.
We can report that two out of three of the products tested - or 165 products in total - didn't pass relevant safety tests.
The full list of everything tested is below - 18 different product types were put through rigorous safety tests. These included everything from carbon monoxide alarms to USB chargers, and for each product test carried out worrying safety results were delivered.
Among the 18 types of products listed above, those tested by Which? were:
Safety tests carried out by other European consumers' associations found teeth-whitening products with too much hydrogen peroxide, cosmetics sold without their ingredients listed and kids' clothes with choke hazards.
Every time products fail Which? safety tests, we inform the online marketplace we purchased them from, along with any other marketplaces listing what appear to be identical products. Each time listings are removed.
But this doesn't mean the products always disappear completely for sale from online marketplaces forever.
New research we've conducted found six listings for fabric child car seats that don't meet EU safety standards. They're the same type of potentially unsafe car seat we've informed certain online marketplace about several times over the past few years.
See why these car seats are so unsafe and what we recently found in our video below.
When we checked the eBay site recently, we found three of these child car seats listed among the cheapest 200 on the site.
On AliExpress, we also found three listings for the same kind of seat.
Amazon and Wish didn't appear to list the dangerous seats when we checked.
Ebay told us: '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's required in our User Agreement that all sellers comply with the law.
'Between October 2018 and October 2019, our filters automatically blocked five million listings from entering the marketplace on product safety grounds. Ebay also works with organisations around the world, including the EU market surveillance authorities and Westminster Trading Standards. If any of these authorities informs us that a product is dangerous, we ban it on all our marketplaces globally and action sellers in the form of a warning, restriction, suspension or ban.'
An AliExpress spokesperson told us: 'AliExpress takes product safety very seriously and works relentlessly to ensure that consumers are protected on our platform. 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.
'When we are made aware of unsafe or prohibited items on our platform, as on this occasion with Which?, we act quickly to remove them and take action against the sellers, which can include suspending their accounts or removing them from the platform.'
A spokesperson for Amazon said: 'Safety is a top priority at Amazon. 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. These are isolated incidents that do not reflect the fantastic products and customer experience provided by millions of small businesses selling in our store.'
A spokesperson for Wish 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.
'As we continue to grow and expand our product range, we are exploring a number of new technologies and initiatives to help improve the customer experience, and ensure we are creating a safe and fun environment for users to shop online.'
As things stand, online marketplaces aren't responsible for the safety of the products sold through their sites, for removing unsafe products from sale or for making sure that customers are informed when something goes wrong with a product.
Some online marketplaces do voluntary checks, but our research and that of consumers' associations across Europe shows that this is inadequate with far too many unsafe products ending up being listed for sale.
Four significant regulatory improvements need to be made to ensure that consumers are protected when buying from online marketplaces.
We want the government to apply the safety requirement in the General Product Safety Regulations, as well as other sector-specific product safety legislation - for products such as toys - to marketplaces. This means the sites will have to enhance their checks before including sellers on their sites.
We're also calling for stronger and more consistent action when unsafe products are identified by enforcement agencies, and a new UK law that will require online marketplaces to make it clear to people they are buying from a trader, rather than another consumer.
Neena Bhati, head of campaigns at Which?, says: '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, so the government must now step in to make online marketplaces responsible for ensuring the safety of the products sold on their sites.'
Additional reporting: Jon Barrow, Matt Knight