"The importance of protecting the population not just from UVB but also from UVA irradiation has profound implications on public health worldwide," said researcher Gary Halliday.
Australian researchers found that UVA rays damaged cells in the deep basal keratinocyte cell layer - the skin layer containing the mother cells that ensure continual regeneration of the skin through cell division - making skin cells migrating out from this region highly vulnerable to cancer.
"The predominance of UVA mutations in the basal cell layer reinforces the pivotal role UVA may play in the malignant transformation of human skin," said Halliday.
Previously experts thought that UBA rays posed less of a cancer risk because, although they penetrate deeper than the shorter wavelength UVB rays that cause sunburn, they were thought to have less impact on DNA.
The study - in Proceedings of the National Academy of Sciences - examined tissue samples from 16 patients being treated for two different types of skin cancer at the Royal Prince Alfred Hospital in Camperdown, Australia.
The researchers claim that both types of cancer studied showed greater signs of basal skin layer damage from UVA rather than UVB rays, with the vast majority of UVB mutations in the outer layers of the skin.
"This is an important paper which adds new data to the growing body of evidence pointing to a significant role for UVA in the development of skin cancer. Although experimentally very thorough it is still a relatively small study, and the findings now need to be confirmed in similar larger scale studies," said Cancer Research UK dermatologist Catherine Harwood.