A nasal spray designed to produce a glowing tan is "potentially dangerous", experts have said.
The Medicines and Healthcare products Regulatory Agency (MHRA) has issued a warning over Ubertan, which is being advertised and sold illegally in UK gyms, beauty salons and online.
Tests carried out on the product have revealed it contains an unlicensed compound called melanotan II, which is similar to a hormone that increases the production of melanin in the skin, causing it to darken.
The MHRA said melanotan II is unlicensed and could have side effects. There is also no evidence proving its safety or even if it works.
Another version of Ubertan tested by the MHRA contained the unlicensed herbal extract forskolin, which has the potential to lower blood pressure and promote secretion of hormones from the thyroid. The MHRA has now written to several online stores and retailers to tell them to stop marketing the product.
Head of the MHRA's medicines borderline section David Carter said: "Putting your health on the line just in order to get a tan is just really not worth the risk. None of these products have been clinically tested and as a result there is no way of knowing just how serious the side-effects could be, or the impact on your health.
"If you have this product at home do not use it again, throw it in the bin. And if you have used it and you have concerns then speak to your GP."
Bevis Man, from the British Skin Foundation, said: "People may think that these products are a safe alternative to sunbeds and sunbathing because it doesn't require exposure to UV light, but the truth is that because we know so little about them, it's probably best to stay away until we find out more.
"All medicines that are used in the UK have to be licensed by the MHRA. Cosmetic self-tanning products should have clear labelling to advise users about how to use them and what they contain, so we would definitely recommend that people think twice before using any product that has no clear labelling.
"There are other ways to get a fake tan without the risks of using an unregulated product, so until it gets the all clear for use on humans, stick to spray tans and self-tanning lotions instead."