High factor sunscreen cannot be relied on to protect against the deadliest form of skin cancer, new research suggests.
The study by Cancer Research UK found that while high factor sunscreen can reduce DNA damage caused by the sun and slow the onset of malignant melanoma, it does not offer complete protection.
It proves public health campaigns are right to promote a combination of sun protection methods such as hats and shade alongside sunscreen, scientists say.
During the study, mice who were predisposed to melanoma took only around 30 per cent longer to develop cancer when coated in SPF 50 sunscreen than those who were not.
The research, published in the scientific journal Nature, revealed that UV light directly damages DNA in the skin's pigment cells, which increases the chance of developing the disease.
Scientists discovered that UV light also causes faults in the gene which normally helps to protect against sun damage.
Professor Richard Marais, study author and Cancer Research UK scientist, based at the University of Manchester, said: "UV light has long been known to cause melanoma skin cancer, but exactly how this happens has not been clear. These studies allow us to begin to understand how UV light causes melanoma.
"UV light targets the very genes protecting us from its own damaging effects, showing how dangerous this cancer-causing agent is. Very importantly, this study provides proof that sunscreen does not offer complete protection from the damaging effects of UV light.
"This work highlights the importance of combining sunscreen with other strategies to protect our skin, including wearing hats and loose fitting clothing, and seeking shade when the sun is at its strongest."
Dr Julie Sharp, head of health information at Cancer Research UK, warned that people exposed to sunlight must wear a cream with good UVA protection.
"We've known for some time that sunscreen, when applied properly, can help protect our skin from the harmful effects of the sun's rays. But people tend to think they're invincible once they've put it on and end up spending longer out in the sun, increasing their overall exposure to UV rays.
"It's essential to get into good sun safety habits ... sunburn is a clear sign that the DNA in your skin cells has been damaged and, over time, this can lead to skin cancer," Dr Sharp said.