Humans are unlikely ever to live beyond the age of 125, say researchers who claim we are already close to the lifespan limit.

The scientists studied survival data dating back to 1900 from more than 40 countries.

They found evidence of increasing average life expectancy, meaning that over time more people lived to a ripe old age.

Babies born in the US today could expect to live nearly to the age of 79, on average. In comparison, average life expectancy for Americans born in 1900 was only 47.

But the same study highlighted how unusual it was to live beyond 100, regardless of the year in which people were born.

The team calculated that 125 was likely to be the absolute limit of human lifespan due to genetic factors.

Lead researcher Professor Jan Vig, from the Albert Einstein College of Medicine in New York City, said: "Demographers as well as biologists have contended there is no reason to think that the on-going increase in maximum lifespan will end soon. But our data strongly suggest that it has already been attained and that this happened in the 1990s.

"Further progress against infectious and chronic diseases may continue boosting average life expectancy, but not maximum lifespan.

"While it's conceivable that therapeutic breakthroughs might extend human longevity beyond the limits we've calculated, such advances would need to overwhelm the many genetic variants that appear to collectively determine the human lifespan

"Perhaps resources now being spent to increase lifespan should instead go to lengthening healthspan - the duration of old age spent in good health."

The study, published in the journal Nature, focused on people living to 110 or older between 1968 and 2006 in the US, UK, France and Japan.

Age at death for these super-centenarians rose rapidly between the 1970s and early 1990s but reached a plateau in the mid-1990s.

French woman Jeanne Calment, who died in 1997 aged 122, achieved the longest documented lifespan of any person in history.

The researchers put the current average life span of the oldest individuals on Earth at 115.