Illegal home testing kits for HIV are giving people incorrect results, the Medicines and Healthcare products Regulatory Agency (MHRA) has warned.
It said there were issues with other tests for sexually transmitted diseases which were legal but might also be inaccurate.
The MHRA is investigating a UK website which is selling the tests.
The Health Protection Agency has written to some of those affected to say the tests are unreliable.
Sales records gathered during the investigation showed that about 500 tests for sexually transmitted diseases - such as HIV, chlamydia and syphilis - had been sold.
It warned that as well as home HIV tests being against the law, the other tests did not meet European regulations.
Susanne Ludgate, MHRA clinical director of devices, said: "We're concerned that there may be a number of self-test kits being sold online that may not be compliant with the relevant piece of legislation and we're urging people not to consider the internet as a method of anonymous testing.
"These kits may be unreliable and there is a significant risk they could be providing the user with a false result.
"The instructions for use might also be incorrect or confusing and not adequate for someone trying to use the kit in their home."
She said people should check for the "CE mark", which shows the tests have been approved.
The Health Protection Agency has contacted those known to have ordered the kits.
Dr Fortune Ncube, from the HPA's blood borne viruses department, said: "If anybody feels they have put themselves at risk they should contact their local GP or go to their most convenient GUM clinic, where they can receive a full screen for all STIs, including HIV.
"Rapid and confidential tests, as well as sexual health advice, are available through the NHS without charge."
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