An over-the-counter painkiller taken by millions of Britons could hold back ageing, research suggests.

In laboratory tests, ibuprofen was found to extend the lives of worms and flies by the equivalent of about 12 years in human terms.

The simple creatures not only lived longer, but seemed to maintain their fitness and health as they aged.

Despite the big evolutionary gap between worms and people, scientists believe they have stumbled on a new aspect to ageing that could have major implications for humans.

Lead researcher Professor Michael Polymenis, from Texas A&M University in the US, said: "We are not sure why this works but it is worth exploring further.

"This study was a proof of principle, to show that common, relatively safe drugs in humans can extend the lifespan of very diverse organisms.

"Therefore, it should be possible to find others like ibuprofen with even better ability to extend lifespan, with the aim of adding healthy years of life in people."

Ibuprofen, like aspirin, is a non-steroidal anti-inflammatory drug (NSAID) used to treat pain and fever.

It was developed by the Boots company in the 1960s and became available over-the-counter around the world in the 1980s.

The World Health Organisation (WHO) includes the drug on its list of "essential medications" needed in every basic health system.

Although considered relatively safe, high doses can have harmful side effects that impact the gastrointestinal tract and liver.

In the new research, scientists exposed three model organisms - baker's yeast, the lab worm Caenorhabditis elegans and the fruit fly - to ibuprofen. The doses used were broadly the equivalent of those taken by humans.

The treatment added about 15% to the lives of the different species, which in human terms amounts to an extra dozen or so years of life.

It was also healthy living time. Both the treated flies and worms appeared healthier in old age than those left untreated, the scientists reported in the online journal Public Library of Science Genetics.

Co-author Dr Chong He, from the Buck Institute for Age Research in California, US, said: "Healthy worms tend to thrash a lot and the treated worms thrashed much longer than would be normally expected.

"As they aged, they also swallowed food much faster than expected."

A possible clue to the effect came when the scientists found that ibuprofen interfered with the ability of yeast cells to pick up tryptophan, an amino acid protein building block found in every organism.

Tryptophan is an essential nutrient obtained in the diet from protein.

Why blocking tryptophan might affect ageing is still unknown. Another member of the team, Dr Brian Kennedy, also from the Buck Institute, said: "There is a lot to be excited about. Not only did all the species live longer, but the treated flies and worms appeared more healthy.

"The research shows that ibuprofen impacts a process not yet implicated in ageing, giving us a new way to study and understand the ageing process."

He added: "Our institute is interested in finding out why people get sick when they get old.

"We think that by understanding those processes, we can intervene and find ways to extend human health span, keeping people healthier longer and slowing down ageing. That's our ultimate goal."

Dr David Clancy, lecturer in genetics and the biology of ageing at Lancaster University, said: "The question is whether this finding might be relevant to lifespan in humans.

"If the lifespan extension was dependent upon early growth or developmental effects, it is unlikely to be useful in humans. Otherwise, it might possibly provide benefits.

"There should already be data from other observational studies in humans that will give a strong indication about whether there is an association in people.

"Ideally, a randomised control trial would give a more robust answer, but due to the need to follow the experiment through the human lifespan this would take too much time."