Stem cells taken from amniotic fluid can be transformed into a more versatile state similar to embryonic stem cells and may offer an alternative to the medically valuable but controversial cells, scientists said on Tuesday.

British researchers said they had succeeded in reprogramming amniotic fluid cells without having to introduce extra genes.

This suggests the possibility that stem cells derived from donated amniotic fluid could be stored in banks and used for medical therapies and in research, they said, offering a less problematic alternative to embryonic stem cells.

Stem cells are the body's master cells, the source for all other cells. Scientists say that by helping to regenerate tissue, they could offer new ways of treating diseases for which there are currently no treatments - including heart disease, Parkinson's and stroke.

Embryonic stem cells are harvested from embryos and have the potential to become almost any type of tissue. Other types of stem cells, including adult or so-called "induced pluripotent" stem cells, are less controversial, but are also less flexible.