The BBC's James Westhead reports: "People who use them tend to stay in the sun longer"

A study, published in the US Journal of the National Cancer Institute, said that people who wore higher factor sunscreens tended to stay out much longer, because they felt protected from the risk of sunburn.

And a British biochemist has suggested that the cocktail of chemicals involved in sunscreens could be converted into "free radical" molecules, which could cause cell damage and lead to cancer.

The US study, revealed on the BBC's Newsnight programme, appears to demonstrate why incidence of skin cancer has increased, even though sunscreens have become popular among fair-skinned people.

Sunscreen advocated

But the Health Education Authority has accused the study's authors of spreading a confusing message, and stressed the importance of sunscreen.

A spokesman said: "Don't stop using sunscreens but do remember that you shouldn't use them so you can sunbathe longer."

Mike Brown of Boots the Chemist and Kate Law of the Cancer Research Campaign

Dr Philippe Autier, a scientist at the European Institute of Oncology in Milan, led the original study.

He found children using sunscreen returned from holiday with more skin moles - a possible sign of increased cancer risk.

He set out to see if this was because sunscreens were making sunbathers overconfident.

The sun can still damage your skin, even if you wear sunscreen

He gave two groups of volunteers different strengths of sunscreen, and found that those using less protective sunscreen spent 25% less time in the sun than those with high-protection creams.

He said: "Consumers should be warned that using a sunscreen may increase the amount of time spent in the sun.

"Because of that it may increase your risk of sun cancer."

Meanwhile, Dr John Knowland, a biochemist at Oxford University, is looking at another reason why sunscreen may be behind increased skin cancer rates.

He said: "The important thing to remember is that a sunscreen that absorbs energy cannot actually destroy that energy, it has to do something with it.

'Cover up'

"The concern that some people have is that they can convert the light energy into chemical energy, which is potentially damaging."

He and his colleagues are trying to model the effects of sunlight on human skin, looking at how the DNA might be changed.

Professor Jack Cuzick, head of the epidemiology unit at the Imperial Cancer Research Fund, said: "Obviously people would be more at risk if they didn't wear any sunscreen but they should not rely solely on sunscreen to protect themselves against the sun.

"If they do use a sunscreen they should make sure it has both UVA and UVB protection.