The consumer watchdog says it found evidence of toys listed for sale on Amazon Marketplace and eBay that appeared to have already been flagged by Safety Gate, the EU's rapid alert system for dangerous products.
This delay is despite both sites claiming to have dedicated teams and systems in place to monitor listings.
Which? presented eBay with 12 products including toy slimes, a Transformers helmet and a cartoon helicopter which all appeared to bear significant similarities to Safety-Gate listed dangerous products either by a shared batch or product number.
Products were deemed unsafe for a range of reasons including high levels of toxic chemicals, volume levels which could harm a child’s hearing or small detachable pieces that could pose a choking hazard to children.
The marketplace subsequently removed all 12 product listings.
Which? also reported six products to Amazon including a magnetic building set, an inflatable swim ring and a remote control car. The products were deemed unsafe for reasons such as the potential to cause an intestinal blockage or perforation, and excessive levels of lead.
Amazon removed five of the products but did not respond to enquiries about a toy dinosaur - Which? says the item concerned had the same model number as a toy flagged for containing too much lead.
Although the items have now - largely - been removed, the watchdog is concerned about the ongoing problem of marketplaces not being accountable for safety.
As a result the consumer group is calling on the government to make online marketplaces such as eBay and Amazon legally responsible for stopping dangerous products from being sold.
Online marketplaces are not currently responsible for ensuring products are safe, removing unsafe products from sale or for notifying customers when something goes wrong.
Caroline Normand, Which? director of advocacy, said: "It's clear that consumer protections have not kept pace with the changes to the retail industry, and it is not acceptable for marketplaces to pass the buck for the responsibility of the items sold on their sites by simply pointing the finger at sellers.”
Which? says by posing as a seller it was able to list a toy on Amazon marketplace, that had been recalled in October last year because it posed a risk of choking or suffocation.
The information provided for the listing included the barcode number of the product listed on the recall database, and even used the same image.
Despite these details, the product remained live on Amazon for two weeks before being removed by the watchdog.
A spokesperson from Amazon told The Independent: “All sellers must follow our selling guidelines and those who do not will be subject to action including potential removal of their account. The products in question are no longer available.
"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."
A spokesperson from eBay said: “We welcome the findings from Which? Between October 2018 and October 2019 our filters automatically blocked 5 million listings from entering the marketplace on product safety grounds. On average, that’s over 13,500 potentially dangerous listings blocked by our systems every day globally.”