About 12,000 fake kits to test for sexually transmitted infections have been seized in the UK in the past four years, figures show.

The Medicines and Healthcare Regulatory Agency (MHRA) has found around 2,000 products available to buy online since last October.

The fake test devices for HIV and other diseases can show negative results when the person is positive, resulting in an individual believing they are infection-free and unknowingly spreading an infection to others.

Health experts said many people bought the products online in order to test for infections such as syphilis and gonorrhoea to avoid the embarrassment of buying them in a pharmacy.

Mahendra Patel, an honorary visiting professor in pharmacy for the University of Bradford, said self-testing kits should never replace seeking the advice of a healthcare professional.

He said some young people in particular could be buying the testing kits online “because of the price, they want to keep it private and there is the convenience of going on the internet, searching for them quickly and buying it on their phones. They don’t understand the quality assurance behind these products, which is the key part, if it is not from a bonafide supplier.”

He added: “It is important the kits are approved and regulated in this country. Your doctor or community pharmacist can provide professional advice on whether you need to be seen by the doctor straight away if you have other symptoms, or other medication your are taking, or you can visit your local sexual health clinic.”

Examples of warning signs that STI devices are fake include instructions printed on a home printer, the absence of a CE safety mark and the item arriving in a Ziploc bag or sandwich bag.

The MHRA is running a #FakeMeds public health campaign to reduce the harm caused by purchasing fake, unlicensed or counterfeit medical products online. It comes after research found that more than half of all medical products bought online are either substandard or counterfeit.

Last month the former Wales rugby player Gareth Thomas, 45, disclosed on Twitter that he was HIV positive and wanted to “break the stigma” around the condition. The Terrence Higgins Trust, an HIV, Aids and sexual health charity, said the day after Thomas’s announcement was its busiest day of the year for web traffic and orders for testing kits.

Lisa Hallgarten, from the young people’s sexual health and wellbeing charity Brook, said people aged under 25 remained one of the groups most at risk of STIs. She said: “Brook offers free, confidential STI testing and treatment as well as contraception, counselling and other sexual health and wellbeing services. Last year we had more than 76,000 visits to our clinics.

“We would strongly advise anyone getting tested to use our online find a service tool to find their local sexual health service. Getting tested is quick and easy, and many STIs can be tested using a self-swab.”

Hallgarten advised those ordering online to verify the status of the source. She said the organisation was working with SH:24, an NHS-backed provider, to give free STI home-sampling kits to disabled people and their partners.

The MHRA said research showed that one in 10 people in the UK bought fake medical products online in the last year.

It said buying from disreputable websites also increased the risk of being a victim of credit card fraud or identity theft.

An MHRA spokesman said: “Medical devices that do not display the CE mark and four-digit number cannot be guaranteed to meet quality and safety standards and could lead to false negative results, potentially leading to STIs or blood-borne viruses to be spread further.

“Always purchase medical devices from a registered pharmacy or reputable retail outlet. If you have any concerns about your health, speak to a doctor or healthcare professional.”