People considering using home HIV tests have been warned to check the kit is properly authorised after officials seized more than 100 "potentially unreliable" products.
The Medicines and Healthcare products Regulatory Agency (MHRA) said that people considering the home-test kits should ensure the product has a CE mark.
The comments come after the regulator seized 114 kits from two UK-based suppliers.
The Hightop HIV/AIDS Home Test Kit does not have a valid CE mark which means the product has not met a number of regulatory requirements concerning test performance, labelling and instructions for use, the MHRA said.
It warned people not to use or buy the Hightop HIV/AIDS Home Test Kit, adding that people who believe they have used one should seek a further HIV test "because the self-test kits may be unreliable and provide false results".
The regulator said that only approved HIV self-testing kits that have met requirements concerning test performance, labelling and directions for use carry a CE mark.
MHRA's director of devices John Wilkinson said: "People who buy a self-test kit online or from the high street should know what they are buying is safe and reliable.
"Make sure the kit has a CE mark and clearly states that it is intended for home self-testing. Don't use a test kit if it's damaged or the seal is broken.
"If you are concerned you may have used an unreliable test kit, speak to your GP, sexual health clinic, pharmacist or other healthcare professional."
Cary James, head of health promotion at the sexual health charity Terrence Higgins Trust, said: "Home self-test kits for HIV and STIs have many benefits, including letting people test in their own space, in their own time, on their terms.
"However we are extremely concerned to see unregulated products on the market and urge anyone considering a test to only use those with a CE mark.
"Anyone taking a test without a CE mark risks their own health and that of others."