They say they have unpicked the structure of telomerase, an enzyme which, when active, helps keep cells in an "immortal" state and they say it is present in more than nine out of ten types of tumours.

Telomerase, an elusive enzyme, is a complex structure which has been under investigation by researchers worldwide for more than 15 years.

The scientists from Philadelphia's Wistar Institute, say their revelation could lead to drugs which will switch the chemical off and possibly convert immortal cancer cells back into mortal ones.

All the cells in the human body have a natural clock, the telomeres, which shorten every time the cell divides and the theory is that by blocking telomerase in this way, immortal cancer cells could be converted back into mortal ones.

According to experts, after a fixed number of divisions in most cells, the telomeres are reduced to a certain length, and the cell cannot continue dividing and as cell division slows down this is also responsible for changes within the ageing body.

They say some cells, such as stem cells within the embryo, use the chemical telomerase to maintain telomere length and many tumours have hijacked the telomerase system to fuel their uninhibited growth.

The team from Wistar team has found a new way to map the structure of the most active part of the chemical and this detailed picture will help provide molecular targets for drugs.

Dr. Emmanuel Skordalakes, an assistant professor in Wistar's Gene Expression and Regulation Program, who led the study, says telomerase is an ideal target for chemotherapy because it is active in almost all human tumours, but inactive in most normal cells, which means that a drug that deactivates telomerase would possibly work against all cancers, with few side effects.

Experts say the research is an important achievement because telomerase controls the evolution of cancers and is a key characteristic of human cancer cells and the research will help the development of new, broadly effective cancer drugs, as well as anti-aging therapies.

In addition to its role in cancer, telomerase has significant implications for the development of therapies to combat aging and other age-related diseases and by finding ways to activate telomerase under controlled conditions, to allow some cells to begin dividing again could result in healthier, younger-looking tissue that lives longer.

The study is published in the journal Nature.