More than 2,000 people die in the UK from skin cancer each year
Rubbing sunscreen into the skin reduces its effectiveness, a study says.
The Restoration of Appearance and Function Trust, which includes experts in sunburn, looked at how sunscreens ward off cancer-causing radiation.
They found rubbing sunscreen on does not offer even protection and letting a white film dry on was much better.
Skin cancer kills more than 2,000 people a year in the UK, the journal, PhotoChemistry and PhotoBiology, reported.
If the cream is rubbed in, the protection is minimal
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Staying cool in the heat
Lead researcher Dr Rachel Haywood said: "Most people prefer to rub the cream into the skin.
"They find it more pleasant and comfortable.
"However, our research shows for the first time that if the cream is rubbed in, the protection is minimal."
The team used left-over skin from plastic surgery operations and recreated intense sun exposure in the lab.
They measured the levels of tissue-damaging particles called free radicals after exposure and found they rose in direct relation to exposure to UVA rays, which is associated to cancer and premature ageing.
They found that when the sunscreen was rubbed in it offered almost zero protection because the cream accumulated in lines and sweat glands and did not offer even protection.
Researchers said rubbing in sunscreen could even put people at higher risk because while it did not protect against UVA rays it did offer resistance to UVB which causes the skin to redden.
They said that could encourage people to stay in the sun for longer.
Sunscreen not enough
Sara Hiom, from the charity Cancer Research UK, said: "The one message people should take from this research is that you cannot rely on sunscreen alone to protect you from skin cancer.
"Sunscreen should be the last - not the first - line of defence against the sun's harmful rays.
"The most important thing is not to burn and we recommend that people should spend time in the shade between 11am and 3pm when the sun is at its height and they should cover up with a T-shirt, hat and sunglasses.
"Factor 15 plus sunscreen with added UVA protection should be applied generously and reapplied often on any exposed skin.
"People with fair skin who burn easily or who have lots of moles or freckles or a family history of skin cancer should be particularly careful in the sun as they are at the highest risk of the disease."
But Colin Hopper, a skin cancer specialist at University College Hospital, said: "Sun cream should be applied generously, and rubbed in carefully.
"Telling people to leave it on in a thick buttery layer is both impractical and unnecessary."