Details of the first hybrid human embryo clone have been released.

The watershed achievement in biotechnology actually happened last November, but more information was revealed on Thursday. It was achieved using a cell from a man's leg and a cow's egg.

The scientists who created the clone see it as a significant step forward in the search for a way of producing human stem cells.

These are "master" cells that can develop into any type of cell - skin, bone, blood, etc. They are believed to have the potential to provide perfect-match tissue for transplantation and the treatment of diseases such as Parkinson's.

Cloning questions

But this development will also see a significant heightening of the debate over the ethics of human cloning and, indeed, what it means to be a human.

Advanced Cell Technology (ACT), a leading, private biotechnology company, made the first human embryo clone and let it develop for twelve days before destroying it. In a normal pregnancy, an embryo implants into the womb wall after 14 days.

Dr Robert Lanza, ACT's director of tissue engineering, told the Daily Mail newspaper that the embryo could not be seen as a person before 14 days. The company said it had released news of the discovery to try to allay fears over the artificial conception of life.

It is believed that many more hybrid embryos have been created in the same way and destroyed since November.

No child clones

ACT said it had no intention of attempting to use the human cloning procedure to start a pregnancy - its aim was "therapeutic cloning" not "reproductive cloning", it said.

Lord Robert Winston, a British fertility expert, said the research was "totally ethical".

But opponents said the development of the technology made the eventual birth of a human clone inevitable. This, they said, would have profound implications for the nature of family relationships, the law and health.

The technology used to create the clone was very similar to that used to make Dolly the sheep clone. Over 200 embryos were used before Dolly finally appeared, showing that cloning is not a well-understood or easy-to-perform technique.

It is believed ACT used a cow's egg. This had its DNA removed and replaced with human DNA. The new cell was then chemically persuaded to behave like a new embryo and start dividing. This is how ACT hope to cultivate stem cells.

But Dr Maisam Mitalipova, a pioneer of this human-cow type of cloning, told the Daily Mail: "We didn't get good quality embryos and so they may not get good quality stem cells."

Another US company, Geron, is also reported to be attempting to make human embryo clones for therapeutic purposes.

It recently bought all the shares in Roslin Bio-Med, a company set up to commercialise the cloning expertise of the Roslin Institute, Scotland, where Dolly the sheep was created.

Geron has not publicly stated whether its attempts have been successful and it may be that ACT achieved the feat first.