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Most people "underestimate the sun" and the irrevocable damage it can do to skin, a 21-year-old skin cancer survivor warned Good Morning Britain.
Kelli Cooke from West Sussex was diagnosed with malignant melanoma at 19, despite always being vigilant with sunscreen and wearing factor 30.
She is now in remission but said she had to "be really careful" and carried suncream with her "all the time".
Slathering on sunscreen will not "make you invincible" to the Sun's rays and there is still a chance of DNA damage from UV, a medical expert said.
Prof Richard Marais, who led research into the protection provided by sunscreens, said:
Powerful sunscreens may not always be enough to prevent malignant melanoma, a deadly form of skin cancer, from developing under the Sun's powerful rays, research has found.
Mice wearing Factor 50 sunscreen were exposed to a weekly dose of UV radiation - similar to that a person would be exposed to if they spent an hour a week in a garden in southern England.
Scientists at Manchester University and London's Institute of Cancer Research found tumours still developed but at a 30% slower rate than the skin cancer that developed in mice who were not wearing protection.
Their paper, published in the journal Nature, revealed that even highest grade SPF 50 suncream allows sufficient UV radiation through to damage the DNA in the skins pigment cells.