Who wants to live forever? Some flatworms do, even if it means no sex.

British scientists have found that a species of flatworm can overcome the process of ageing to become potentially immortal and say their work sheds light on possibilities of alleviating ageing and age-related characteristics in human cells.

In a study published in the Proceedings of the National Academy of Sciences journal on Monday the researchers found that the flatworms, known as planarian worms, can continuously maintain the length of a crucial part of their DNA, known as telomeres, during regeneration.

Our data satisfy one of the predictions about what it would take for an animal to be potentially immortal, said Aziz Aboobaker, who led the research at Britain's University of Nottingham. The next goals for us are to understand the mechanisms in more detail and to understand more about how you evolve an immortal animal.

Planarian worms have long fascinated scientists because they have an extraordinary ability to regenerate. A planarian worm split lengthwise or crosswise will regenerate into two separate living worms.

Aboobaker's team studied two types of planarian - those that reproduce sexually, like humans, and those that reproduce asexually by simply dividing in two.

Both types appear to regenerate indefinitely by growing new muscles, skin, guts and even entire brains again and again, Aboobaker explained in a statement about the work, but the asexual ones also renew their stocks of a key enzyme which may mean they can be immortal.

Scientists know that one of the key factors associated with ageing cells is telomere length. Telomeres are sections of DNA that cap the ends of chromosomes, protecting them from damage and the loss of cell functions linked to ageing. Shorter telomeres are thought to be an indicator of faster ageing.

Previous research -- which won the Nobel Prize for Medicine in 2009 -- has shown that telomeres can be maintained by the activity of an enzyme called telomerase.

In most sexually reproducing organisms the enzyme is most active during early development, but Aboobaker's team found that in the asexual worms, the planarian version of the enzyme is dramatically increased during regeneration - a factor that allows stem cells to maintain their telomeres as they divide to replace missing tissues.

Douglas Kell, chief executive of the Biotechnology and Biological Sciences Research Council which part-funded the study, described the finding as exciting and said it contributes significantly to our fundamental understanding of some of the processes involved in ageing.

The work also builds strong foundations for improving health and potentially longevity in other organisms, including humans, he said in a statement.