These cells have the potential to become any tissue in the body and scientists believe they will eventually lead to powerful new treatments for a host of medical conditions, including diabetes and Parkinson's Disease.

And producing the cells via cloning allows the possibility of creating perfect match tissues in transplants.

Geron Corporation, at Menlo Park in California, has admitted it is trying to create the first batches of cloned human embryos. At the same time, Advanced Cell Technology, based in Massachusetts, is reportedly using a technology that attempt to fuse cells from people with cows' eggs.

Harvest "unethical"

Scientists see embryos as a source for stem cells

But for those who believe life begins at the time of conception, the new research is hard to accept. Some fear a programme of harvesting human embryonic cells could eventually lead to the wholesale creation of a cloned human.

Mark Nicholls, operations manager of the British-based Movement Against The Cloning of Humans (Match) says its not even necessary: "Advocates of spare part human cloning say that the work could lead to new treatments for many illnesses.

"However, what they do not say is that the body of a living adult also contains stem cells that could be used for therapeutic purposes, making redundant the need to destroy human embryos," he says.

Benefits outweigh concern

Michael West, president of ACT, is quoted by the Washington Post as having little patience with such arguments: "I think people don't realise that we're talking about cells that have not become anything yet. To prevent science from using cells to cure human diseases would be a horrific step backwards."

Dolly was produced from reprogrammed genetic material

In May, Geron bought all the shares in Roslin Bio-Med, the commercial arm of the Scottish laboratory which produced Dolly the cloned sheep, and have promised to invest £12.5 million over the next six years.

The two companies hope to combine Geron's expertise in stem cells and Roslin's experience with animal cloning.

They hope to discover how adult cells can be reprogrammed to become different types of cell, again so that they can be used in transplants.