The Medicines and Healthcare products Regulatory Agency says the Covid pandemic has created demand for devices that screen for a raised temperature.
Accurate ones should carry a CE or UKCA mark, it advises.
But people should also be aware that temperature screening is not a reliable way to detect Covid.
Anyone who thinks they may be infected should have a swab test.
The main symptoms of Covid, says the NHS, are:
* a high temperature
* a new, continuous cough
* a loss or change to your sense of smell or taste
Some people have other symptoms too, such as fatigue, muscle aches and a headache, but others can have none and still be infected.
In July 2020, the MHRA issued a warning to manufacturers and suppliers of thermal cameras which cautioned them against making claims which directly relate to Covid diagnosis.
MHRA spokesman Graeme Tunbridge said: "People in the UK are unknowingly buying poor-quality infrared thermometers which do not meet the required standards of safety and performance.
"It is important that people know what to look out for when purchasing these products, or any other medical devices online. You should always look for the CE or UKCA mark on any medical device - this will either be on the device or on its packaging, and that includes when you are buying these products online."