Stem cells that have been grown to cure disease might have the reverse effect and turn cancerous if they are allowed to develop for too long outside the body.

Two new studies have shown that adult stem cells, such as those taken from bone marrow, can be harmful if they are allowed to divide for too long.

Scientists already know of a possible cancer risk from stem cells taken from very early embryos, but the new research shows adult stem cells are safe only if the number of times they are allowed to divide outside the body is limited.

Researchers at the Autonomous University of Madrid grew human stem cells extracted from fat tissue for up to eight months.

In that time the cells divided between 90 and 140 times. When they were transplanted into animals, the oldest cells formed cancers.

Antonio Bernad, who is leading the research team, told the New Scientist magazine that pioneering treatments which already test bone marrow stem cells in humans should be safe because the cells are grown outside the body for a short time.

"In normal conditions in clinical applications we think the cells are pretty safe, but we must be careful, he said. "The key is not to grow them for too long."

A Danish research team found that the reason stem cells become cancerous may be because after a time they start to make an enzyme that activates the "fuse" on chromosomes which normally limits the number of times cells can divide.

If the division of stem cells is unlimited, they will eventually turn cancerous.

Robert Lanza, of Advanced Cell Technology, a US stem cell company, said the research highlighted the need for rigorous safety measures. He told the New Scientist: "Both these papers reinforce the potential danger of using stem cells."