The pioneering research overcomes one of the most pressing ethical issues faced by stem cell scientists today - that the most promising variety of stem cells known can be obtained only from embryos, which are cannibalised when the cells are collected.

Researchers claim the new technique can produce stem cells without harming embryos. In a series of papers published today, they describe experiments in which skin cells were plucked from adult mice and "reprogrammed", turning them into cells that are indistinguishable from embryonic stem cells, the only kind thought to be able to grow into all of the body's tissue types.

The breakthrough is expected to overcome a major hurdle that has severely hampered progress in stem cell research. For any stem cell treatment to be useful, it has to work without causing an immune reaction in patients. This means scientists must either create enormous stockpiles of different stem cells, which can then be matched to patients, much as organs are before transplant operations. Or, scientists have to perfect a technique called therapeutic cloning, in which a cell taken from a patient is cloned to produce stem cells that are an exact genetic match. Neither is simple.

The research points to a new way around the problem, suggesting it may be possible to take any cell from a patient and convert into an embryonic stem cell. This could then be used to grow batches of other tissues for future operations, such as neurons and heart cells.

Two teams, one lead by Shinya Yamanaka at Kyoto University and the other by Rudolf Jaenisch at the Whitehead Institute at Massachusetts Institute of Technology, announced details of their research in the journal Nature today.

Both groups took skin cells from adult mice and injected them with a harmless virus that was genetically modified to carry four carefully selected genes, known as Oct4, Sox2, c-Myc and Klf4. Together the genes rewound the clock on the adult cells, returning them to a youthful state in which they could then grow into any other cell type.

"These reprogrammed cells, by all criteria that we can apply, are indistinguishable from embryonic stem cells," said Professor Jaenisch.

To prove the new cells could form any tissue type in the body, they tagged some and injected them into early-stage mouse embryos. When the mice were born, the cells were spotted throughout and were even passed on through sperm and eggs to the next generation.

The technique is not yet safe enough to use in humans, not least because two of the genes injected into the cells increase the risk of cancer. Researchers at other laboratories aim to get around this problem by reprogramming cells with substances that mimic the genes for only as long as they are needed.

"Reprogramming cells in this way is one of the biggest breakthroughs we've seen in the field," said Steven Pollard, a stem cell scientist at the Wellcome Trust centre for stem cell research in Cambridge.

Lyle Armstrong, whose team at Newcastle University is awaiting permission to create stem cells from embryos made by fusing animal and human cells, said the research would give scientists a chance to understand how the biological clocks of cells can be reset.

But Paul de Sousa, a stem cell expert at Edinburgh University's Scottish Centre for Regenerative Medicine, said the cells might still be able to treat patients with terminal diseases even if they do contain cancer-causing genes. "When people are suffering from a degenerative disease, injecting them with cells is already a last option to save their life or improve their quality of life. It's all about control, knowing that a cell is going to do what you want it to do and not something else."