Scientists in the United States announced on Thursday that they had succeeded in isolating and growing human embryonic stem cells - the parent cells for all the tissues in the body.

Their discovery could revolutionise medicine, offering the possibility of growing new organs to order for transplant surgery.

But because the process uses fertilised eggs it has caused a controversy over ethics.

Professor Scarisbrook: "they are fellow human beings"

Professor Jack Scarisbrook from the British anti-abortion group Life, says the cells used are potential humans and the practice should be banned.

"They are not eggs any longer they are fellow human beings and those human beings haven't given their consent. Civilised society says that we must not use human beings without their consent in experimentation."

The donors of the eggs had given consent.

The scientists also insist that the cells could not be used to clone a human - another of the many ethical issues that this type of work raises.

Living in a cell

It was the birth of Dolly the sheep - the first clone of an adult mammal - in Britain in January 1996, that fired the debate over the possibility of using the same technique on humans.

While it is unethical to some, others are quick to point out what they say are the benefits of experimenting with human cells.

Dr Smith on the future treatment of patients

Dr Austin Smith, Director of Genome Research in Edinburgh, describes the possible future benefits.

"Ultimately, for every individual early in their life ... a sample of tissue can be taken and used to establish a stem cell culture.

"This will be frozen down and available throughout their life. If they have any kind of illness or injury, their own cells will be there, to be grown up and produce the particular kind of cells to treat their disease."

Transplanting dysfunctional cells could cure Parkinson's Disease and diabetes

The scientists from University of Wisconsin-Madison, one of the teams that carried out the research, say they are currently able to grow a number of different cells, including muscle, cartilage and neurones, which can be used for repairing organs but not making them.

One of the researchers, Dr James Thompson, says studying the human cell is the best approach to understand human development.