Health officials have warned the public not to inject a synthetic hormone, which is sold illegally online, to artificially top up their tans.

The Medicines and Healthcare products Regulatory Agency (MHRA), which regulates medicines and ensures they are safe to use, said more and more British people have begun using Melanotan, a synthetic hormone that encourages the body to produce melanin, the body's natural protection from the sun, which also darkens the skin.

The drug, which has to be injected into the skin and is illegally sold online or under the counter in gyms and beauty salons, has not been officially safety-tested by any Western governmental healthcare agency. Doctors fear that because the drug is self-injected, users of Melanotan are putting themselves at risk of infections such as hepatitis or HIV.

The drug is available in two forms – Melanotan I, which helps produce a tan, and Melanotan II, which is also known to boost libido. Under UK law it is illegal to ship or sell both variants of the hormone but it is not illegal for customers to buy it.

The Independent has discovered one salon in London publicly advertising Melanotan on an online forum. Urban Wellbeing, a tanning salon in Soho, placed an advert on a forum saying it was a "leading supplier of the personal research peptide Melanotan II, with a London-based agent". When telephoned by The Independent yesterday an employee claimed the hormone was sold through an independent agent, not through the shop.

David Carter, of the MHRA, warned people not buy Melanotan. "Don't be fooled into thinking that Melanotan offers a shortcut to a safer and more even tan. The safety of these products is unknown and they are unlicensed in the UK. The side-effects could be extremely serious."