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Applying caffeine to the skin in sunny weather may protect against a type of skin cancer, US researchers hope.

Experiments on mice, reported in the , suggest that caffeine prevents UV damage.

Tumours took longer to develop and there were fewer of them, the study reports.

But other experts warned there could be an adverse affect on other cancers by using caffeine in this way.

The researchers did not apply caffeine to the mice, instead they genetically modified the animals.

Caffeine is known to interfere with a protein involved in detecting DNA damage called ATR. Scientists modified the mice so they did not produce any ATR in the skin.

Mice were then exposed to UV light three times a week for 40 weeks.

Those without ATR developed their first tumour three weeks later than normal mice. After 19 weeks there were 69% fewer tumours in mice without ATR, although by the end of the study all of the animals had developed one of the most common forms of skin cancer - squamous cell carcinoma.

Prevention possibility

The study said: "At any given point in time the average number of tumours in [modified] mice was significantly lower.

"Caffeine application could be useful in preventing UV-induced skin cancers," the researchers add.

Applying caffeine on a beach to a person's skin, however, is not the same thing as genetically modifying mouse skin in a laboratory. Further studies would be needed to show any possible protective effect in humans.

Prof Dot Bennett, professor of cell biology, St George's, University of London, said: "The authors suggest adding caffeine or related molecules to sunscreens.

"First one might want to check there is no adverse effect of caffeine on the incidence of other cancers, especially melanoma, pigmented skin cancer, which kills over four times as many people as squamous cell carcinoma.

"But caffeine lotion might promote tanning a little, since this family of molecules stimulates pigment cells to make more pigment."