More than 80% of items bought from online marketplaces have failed safety tests.
The category with the most problems was toys, followed by small mains-powered electricals.
The Office for Product Safety and Standards (OPSS) tested 2,260 products sold via online marketplaces between October 2021 and September 2022.
It found that 1,832 of them - 81% - failed to meet safety standards.
Some of the toys failed because they were considered to be strangulation hazards for children under 36 months of age.
Electrical goods failed because they posed a risk of electric shocks or electrical fires.
Cosmetics were also tested, as were products containing button batteries which routinely appeared in recalls and alerts, had poor reviews or appeared to be of low quality in the online photos.
The Chartered Trading Standards Institute said the "sheer volume and proportion of goods that aren't meeting basic safety checks is appalling".
Its chief executive, John Herriman, added: "It is concerning to see this level of products failing safety testing, particularly when the highest category of non-compliance is toys."
Lesley Rudd, chief executive of safety charity Electrical Safety First, said the evidence showed that "online marketplaces are a hotbed for dangerous and non-compliant goods".
She continued: "Decisive action has never been so urgent. Online marketplaces cannot be relied upon to self-regulate the issue of illegal and harmful goods on their platforms.
"The government must act without delay to finally force them to take steps to ensure that these goods sold via their sites, from which they profit, are safe."
Jerry Burnie, head of compliance at the British Toy and Hobby Association (BTHA), said: "The BTHA have tested over 550 products in the last six years and have been pushing for a change in the law to ensure that the online marketplaces are jointly and severally liable for products sold by third parties via their platforms, which otherwise would not have access to the UK market.
"This is a situation that has not improved over those six years. We are pleased that the OPSS's own figures have now confirmed our findings and hope that sufficient action will be taken to ensure consumers are protected from these dangerous and unsafe products."