Scientists have developed a new drug that mimics sunlight to make a person's skin tan without being exposed to harmful ultraviolet radiation, reducing the risk of skin cancer.

In tests on human skin samples and mice, the drug tricked the skin into producing the brown form of the pigment melanin, that gives human skin, hair and eyes their colour.

According to researchers at Massachusetts General Hospital in the US, the finding could prevent skin cancer and even slow the ageing process.

Ultraviolet (UV) light makes the skin tan by causing damage. This kicks off a chain of chemical reactions in the skin that ultimately leads to dark melanin - the body's natural sunblock - being made.

The new drug is rubbed into the skin to skip the damage and kick-start the process of making melanin.

"It has a potent darkening effect. Under the microscope it's the real melanin, it really is activating the production of pigment in a UV-independent fashion," David Fisher, one of the researchers, told BBC News.

It is a markedly different approach to fake tan, which "paints" the skin without the protection from melanin, sun beds, which expose the skin to UV light or pills that claim to boost melanin production but still need UV light.

The scientists want to combine their drug with sun-cream to give maximum protection from solar radiation.

"Our real goal is a novel strategy for protecting skin from UV radiation and cancer. Dark pigment is associated with a lower risk of all forms of skin cancer - that would be really huge," said Fisher.

The research was published in the journal Cell Reports.