Chemicals in oily fish destroy cells in skin and oral tumours while leaving healthy ones alone, scientists said.
Experiments found the omega-3 fatty acids stopped them growing and induced cell death in both early and late stages of the diseases.
The finding could even lead to the development of aerosols or gels containing the molecules that zap skin and mouth cancers.
Professor Kenneth Parkinson, of Queen Mary, University of London, said: “We found the omega-3 fatty acid selectively inhibited the growth of the malignant and pre-malignant cells at doses which did not affect the normal cells.
“Surprisingly, we discovered this was partly due to an over stimulation of a key growth factor (epidermal growth factor) which triggered cell death. This is a novel mechanism of action of these fatty acids.”
The finding published online in the journal Carcinogenesis suggests they could be used in both the treatment and prevention of certain skin and oral cancers.
Omega-3 polyunsaturated fatty acids cannot be made by humans in large quantities so we must acquire them from our diet. As well as oily fish, flax seeds and walnuts are also a good source for the molecules that also ward off heart disease.
The researchers were studying a particular type of cancer called squamous cell carcinoma (SCC).
Squamous cells are the main part of the outermost layers of the skin, and also occur in the lining of the digestive tract, lungs and other areas of the body.
Oral squamous cell carcinomas (OSCC) are the sixth most common cancer worldwide.