For some time, scientists have thought that the ageing process is governed by tiny structures found at the ends of the chromosomes called telomeres.

Chromosomes are the inherited lengths of DNA that contain genes.

It may be possible to extend the duration of healthy adult life using medical interventions that maintain telomere length Dr Richard Cawthon

Each time a cell divides, its telomeres become shorter, until they reach the point where they are so short that no more divisions can take place.

It is thought that this failure to continue to divide, and thus reinvigorate tissue is behind the ageing process.

Now US scientists, working on 20-year-old blood samples, have come up with strong evidence to back up this theory.

They have found that telomere length is a good indicator of whether a person is likely to live for 15 years or more once they reach the age of 60.

Age-related disease

People with shorter telomeres were found to be at greater risk of developing age-related diseases, and were nearly twice as likely as others to die over the next 15 years or so, especially from heart disease and pneumonia.

A team led by Dr Richard Cawthon, from the University of Utah, Salt Lake City, studied 143 people over the age of 60.

They found those with telomeres in the top half for length lived for four to five years longer than those whose telomeres were among the shortest half.

People with the shortest telomeres were three times more likely to develop heart disease, and over eight times more likely to get an infectious disease, particularly pneumonia.

Their rates of death from stroke and cancer were also higher, but by too little to be considered meaningful.

Dr Cawthon said: "This is the first research study showing that telomere length is predictive of survival in humans.

"It supports the hypothesis that telomere shortening is a fundamental process of ageing, contributing to mortality from multiple age-related diseases.

"If this is correct, then it may be possible to extend the duration of healthy adult life using medical interventions that maintain telomere length."

Dr Woodring Wright, of the University of Texas Southwestern Medical Center at Dallas, described the result as impressive, but warned that the findings had to be replicated in other studies before firm conclusions could be drawn.

The research is published in The Lancet.