A drug that reverses ageing might be on the market in just three years, according to scientists.

The drug, which promotes DNA repair and could help astronauts travel to Mars by protecting them from cosmic radiation, could be tested on clinical trial patients within six months.

Working with two biotech companies, researchers have seen promising results in mice in early experiments with the drug, named nicotinamide mononucleotide (NMN).

Lead scientist Professor David Sinclair, from the University of South Wales in Australia and Harvard Medical School in the US, said: "The cells of the old mice were indistinguishable from the young mice, after just one week of treatment.

"This is the closest we are to a safe and effective anti-ageing drug that's perhaps only three to five years away from being on the market if the trials go well."

Nasa is interested in the work as the space agency is looking for ways to protect astronauts from radiation on voyages to Mars.

Without protection, the chance of the astronauts getting cancer is almost 100 per cent.

The first clinical trial is expected to get under way at Brigham and Women's Hospital in Boston, US, this year.

How it works:

NMN boosts levels of NAD+, the oxidised form of the chemical nicotinamide adenine dinucleotide, which is naturally present in every cell of the body and helps regulate protein interactions that control DNA repair.

Accumulated DNA damage is believed to be a major driver of natural ageing and a primary cause of cancer.

Levels of NAD+, a "co-enzyme" or "helper" chemical that assists essential proteins, decline with age.

Recent work published has resulted in an influx of NAD+ supplements online but there is no hard evidence low doses have any effect.

The new research, reported in the journal Science, showed that NAD+ boosts the activity of a well-known DNA repair enzyme called PARP1.

Reduced levels of NAD+ with age were thought to reduce the ability of PARP1 to repair damaged DNA.