A quarter of healthy skin cells could harbour cancer-causing mutations resulting from Sun exposure, a new study by researchers in the UK suggests.

The finding offers clues about the transition of cells from a healthy to a cancerous state, and demonstrates how normal tissue analysis can provide clues about the origins of the disease.

It was previously thought that tumour-forming cell mutations were rare, and occurred just before the onset of cancer.

But this latest study, which examined healthy eyelid skin that was removed from four patients during cosmetic surgery, has revealed that these cancer-linked mutations "are staggeringly common in normal skin," one of the team, oncologist Philip Jones from the University of Cambridge, told Tina Hesman Saey at Science News.

The researchers performed ultra-deep gene sequencing on 234 biopsies of skin taken from the four patients, who were aged between 55 and 73. They were scanning for 74 known mutations.

They found that about 25 percent of skin cells carried at least one mutation linked to cancer. This equated to more than 100 potentially cancer-causing DNA mutations in every 1 square cm (0.1 square inch) of skin, which is roughly the size of a fingernail.

"The most surprising thing is just the scale, that a quarter to a third of cells had these cancerous mutations is way higher than we'd expect, but these cells are functioning normally," senior author Peter Campbell, head of cancer genetics at the Wellcome Trust Sanger Institute, told James Gallagher at the BBC.

Skin cancer is caused by overexposure to the Sun's radiation. The mutations observed in the analysed skin samples showed the patterns associated with the most common and treatable form of skin cancer, known as cutaneous squamous cell carcinoma, rather than the more lethal form, melanoma.

The team found that these mutated cells had grouped into clusters - or 'clones' - and had grown to be around twice the size of similar clusters consisting of normal cells.

But interestingly, none of the mutations uncovered had developed into cancer.

"The burden of mutations observed is high but almost certainly none of these clones would have developed into skin cancer," lead author, IƱigo Martincorena from the Wellcome Trust Sanger Institute, said in a press release.

"Because skin cancers are so common in the population, it makes sense that individuals would carry a large number of mutations. What we are seeing here are the hidden depths of the iceberg, not just the relatively small number that break through the surface waters to become cancer."

The findings, which have been published in the journal Science, could inform the development of new cancer drugs targeting changes in the cell-life cycle, and also highlight the importance of keeping your skin protected from sunlight.

"Whilst the body's immune system can prove quite effective at removing mutated cells, it is important to remember that some of cells aren't removed and mutate into cancers," Bav Shergill from the British Association of Dermatologists, who wasn't involved in the study, told the BBC. "Prevention is the first line of defence."