Scientists have studied the DNA damage caused by ultraviolet rays on the skin of volunteers with various levels of sunscreen thickness.
Their study showed that if applied with less than the recommended two milligrams per square centimetre of skin, sunscreen is not as effective.
However, many people spread the lotion much more thinly than that, as little as 0.8 milligrams per square centimetre.
At that level, an SPF 50 sunscreen would only provide up to 40% of the expected protection.
Samples from participants showed that repeated exposure to UV light caused “considerable” damage to unprotected skin, even if the radiation was low.
The damage was less as the thickness of the sunscreen increased.
Lead researcher Professor Antony Young, from King’s College London, said: “There is no dispute that sunscreen provides important protection against the cancer-causing impact of the sun’s ultraviolet rays.
“However, what this research shows is that the way sunscreen is applied plays an important role in determining how effective it is.
“Given that most people don’t use sunscreens as tested by manufacturers, it’s better for people to use a much higher SPF than they think is necessary.”
Nina Goad, from the British Association of Dermatologists, said people should use an SPF 30 or higher.
She added: “In theory an SPF of 15 should be sufficient, but we know that in real-world situations we need the additional protection offered by a higher SPF.
“It also shows why we shouldn’t rely on sunscreen alone for sun protection, but we should also use clothing and shade.
“An extra consideration is that when we apply sunscreen, we are prone to missing patches of skin, as well as applying it too thinly.”
The findings are reported in the journal Acta Dermato-Venereologica.