Moisturisers used by millions every day may be increasing the risk of common skin cancers, scientists have warned.

Most such creams have never been tested for their cancer-causing effect on the skin. Now scientists have found that they increase the carcinogenic effect of sunlight in mice.

The skin cancers involved are common in humans. Although mostly non-fatal and easily removed, deaths do occur, especially from squamous cell cancers. These are distinct from melanoma, the less common form of skin cancer, which causes over 1,000 deaths a year in the UK but was not the subject of the research.

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Experiments on mice had shown that when caffeine was given orally or applied direct to the skin, it appeared to inhibit cancer. Scientists at Rutgers University, New Jersey, planned to test caffeine as a cancer preventive in humans by adding it to a common moisturiser, Dermabase. Before starting the study they decided to test Dermabase's carcinogenic activity.

To their surprise, they found that it increased the production of tumours in mice that had previously been exposed to ultraviolet light. They then tested three other common moisturisers, all of which increased the production of tumours by an average of 69 per cent.

The significance of the findings for humans has still to be established, the team reports in the Journal of Investigative Dermatology.