Dodgy smoke alarms bought online from sites such as eBay and Wish could be putting lives at risk, consumer watchdog Which? has warned.
Four alarms widely available online completely failed to go off in every test carried out by the group, which branded the results "extremely concerning" and urged anyone who has bought one to stop using it immediately.
All of the alarms were unbranded, made in China and sold through Chinese re-sellers on eBay and Wish. Two came with instructions that were not in English, meaning that it is illegal to sell the product in the UK.
Which? said it was "disturbing" that three of the alarms even carried CE marks, which suggested to consumers they met all safety standards despite the tests suggesting they would be “useless” in a real fire and could put lives at risk.
In a snapshot study, the watchdog found the four alarms accounted for 171 of the cheapest 500 listings for smoke detectors on eBay.
On Wish, 28 out of 200 listings were for three of the alarms.
EBay has now removed listings for the four alarms - though Which? noted it was not the first time it had alerted eBay to faulty alarms.
In 2018, the online marketplace removed 100 listings for smoke alarms which appeared identical to a product that failed seven out of eight fire tests conducted by Which?.
However, in May the watchdog found 60 listings for what appeared to be the same alarm among the cheapest 500 listings on the site.
Which? said it was passing its findings to the Office for Product Safety & Standards (OPSS) as the latest example of the widespread availability of unsafe products on online market places.
The group is calling on eBay and Wish to contact everyone who has purchased one of these products to alert them and to explain how they can get their money back.
Natalie Hitchins, Which? head of home products and services, said customers would be "shocked" to learn that listings for unsafe products were able to simply reappear on sites after being removed.
She accused eBay and Wish of not taking customer safety seriously enough.
“It’s extremely concerning that eBay and Wish are listing smoke alarms that simply do not work, which could put people’s lives at risk in the event of a fire," she said.
"Anyone who has one of the products highlighted should stop using it immediately.
"EBay and Wish must take the safety of their customers more seriously and stop these alarms from getting on to their sites in the first place.
"The safety of products bought on some online marketplaces is becoming an increasing concern. The OPSS and Trading Standards need to take a more proactive approach to ensure that potentially lethal products don’t enter people’s homes through the back door.”
Martyn Allen, from the charity Electrical Safety First, urged people to only buy electrical goods directly from a reputable manufacturer and retailer, as they carry out rigorous tests to ensure products are safe.
He added: "Consumers should take into account that when purchasing a smoke alarm and any other electrical product from an online marketplace they are generally purchasing from a third-party seller, leaving them vulnerable to being exposed to substandard products."