High factor sun cream cannot be relied on to protect against the deadliest form of skin cancer, new research has suggested.
The study by Cancer Research UK found that while high factor sun cream 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 sun cream, scientists said.
During the study, mice who were predisposed to melanoma took only around 30% longer to develop cancer when coated in SPF 50 sun cream 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."
Prof Marais concluded: "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.
She said: "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.
"This research adds important evidence showing that sunscreen has a role, but that you shouldn't just rely on this to protect your skin.
"It's essential to get into good sun safety habits, whether at home or abroad, and take care not to burn - sunburn is a clear sign that the DNA in your skin cells has been damaged and, over time, this can lead to skin cancer.
"When the sun is strong, pop on a t-shirt, spend some time in the shade and use a sunscreen with at least SPF15 and good UVA protection."