The latest study reports a way of delivering foreign genes to reprogramme the cells without using viruses in mouse and human cells.

Efficiency

Study leader Dr Keisuke Kaji, from the Medical Research Council Centre for Regenerative Medicine at the University of Edinburgh, said nobody, including himself, had thought it was really possible.

"It is a step towards the practical use of reprogrammed cells in medicine, perhaps even eliminating the need for human embryos as a source of stem cells," he said.

But he added they needed to improve the efficiency of the process.

Co-author Professor Andras Nagy, from the University of Toronto, added: "We hope that these stem cells will form the basis for treatment for many diseases and conditions that are currently considered incurable."

Professor Sir Ian Wilmut, director of Edinburgh centre where the research was done and the creator of Dolly the sheep, said it would still take time before these cells could be given to patients.

"Crucially, we need to have a method to generate the desired cell types from these stem cells.

"But I believe the team has made great progress and combining this work with that of other scientists working on stem cell differentiation, there is hope that the promise of regenerative medicine could soon be met."

Professor Robin Lovell-Badge, head of the MRC National Institute for Medical Research, said the research was an exciting step in the right direction but there was still a long road ahead.

"For the time being I think it rather premature to suggest that their work will completely remove the need to derive human stem cells from embryos."

He added there was still a lot to learn from human embryonic stem cells in order to know whether stem cells reprogrammed from adult cells are truly useful or not.

Josephine Quintavalle, of Comment on Reproductive Ethics, said: "This is ethical stem cell research at its best, with embryonic-type stem cells derived successfully from adult tissue without involving human embryos.