Nematode worms are a very basic life form

In the study, detailed in journal Nature, nematode worms were exposed to 88,000 chemicals in turn and mianserin extended lifespan by almost a third.

The drug seems to mimic the effects on the body of the only known animal long-life regime - virtual starvation.

Experts said the findings might point to there being genes in humans that could be targeted to increase lifespan.

It may be possible to identify additional genes important in ageing

Dr Linda Buck, Fred Hutchinson Cancer Research Center

For a little-understood reason, depriving the body of all but the minimum amount of calories needed to survive seems to enhance longevity.

However, this is not a practical solution for most people in the modern world and the team from the Fred Hutchinson Cancer Research Center, in Seattle, believe they may have found an easier way to achieve the same effect.

Ideal subjects

While research using nematode worms might not seem to have much relevance to us, in many key ways they are similar, with a central nervous system and sexual reproduction.

In addition, they live for only a matter of weeks, making them ideal subjects for the study of lifespan.

A vast screening programme found that most chemicals had zero impact on the lifespan of the worms, but the antidepressant drug boosted their lives by an average of 30%.

More interestingly, it may be doing this by mimicking the effects of starvation in the brain.

"We don't have any explanation for this," said Dr Linda Buck, one of the researchers.

"All we can say is that if we give the drug to calorific-restricted animals, it doesn't increase their lifespan any further.

"That suggests the same mechanism may be involved."

Food source

She said that it was possible the drug was disturbing the balance of two brain chemicals which help the nematode decide whether there is enough food around to justify laying eggs.

This, she said, might produce a "perceived, but not real" state of starvation.

Dr Buck said that finding a chemical that increased lifespan in animals might point to genes in humans that could be targeted to do the same.

"It may be possible to identify additional genes important in ageing.

"In addition, the chemical approach could point to drugs suitable for testing in mammals."

Professor Janet Lord, from Birmingham University, and a former chair of the British Society for Ageing Research, said: "What's exciting is that the lifespan extension effects we see extend right across the species, from worms and fruit flies to mammals.

"At the moment, there are interesting experiments in primates which, although the animals involved are only middle-aged so far, suggest that calorific restriction does have an effect on longevity."