One in five carbon monoxide detectors are "dodgy", according to tests.
Sixteen detectors were put through lab testing based on EU safety standards - three of which were found to be inefficient.
Detectors made by ATZ Saviour, Binwo and Mudder failed to go off 26 times during 72 carbon monoxide tests, consumer group Which? said.
Despite claiming they complied with the EU safety standard, Which? reported that these devices failed more than one in three of the tests.
The group is warning consumers not to purchase alarms by these brands, adding that all 13 alarms from well-known brands passed all of the gas tests and all carry the BSI Kitemark.
Amazon and eBay have removed the three defective alarms from sale and Which? has alerted Trading Standards and The Council of Gas Detection and Environmental Monitoring of its findings.
Which? editor Richard Headland said: "It's shocking to uncover the sheer volume of carbon monoxide detectors out there that simply don't work.
"Consumers need to be aware that there are dodgy detectors on the market and should only buy products bearing the BSI Kitemark".