By Fergus Walsh
BBC News, Medical correspondent
UK scientists have applied for permission to create embryos by fusing human DNA with cow eggs.
The Newcastle team is pioneering stem cell work
Researchers from Newcastle University and Kings College, London, have asked the Human Fertilisation and Embryology Authority for a three-year licence.
The hybrid human-bovine embryos would be used for stem cell research and would not be allowed to develop for more than a few days.
But critics say it is unethical and potentially dangerous.
Liberal Democrat MP Dr Evan Harris - a member of the Commons Science and Technology Select Committee - said: "If human benefit can be derived by perfecting therapeutic cloning techniques or from research into subsequently-derived stem cells, then it would actually be immoral to prevent it just because of a 'yuck' factor."
Stem cell research is one of the most promising areas of medical science.
Stem cells are the body's master cells and five-day-old embryos are packed with them - each with the potential to turn into any tissue in the body.
It is this ability which scientists want to harness to treat diseases such as Parkinson's Disease, strokes and Alzheimer's Disease.
To do that, they say they need to have access to thousands of embryos for research.
The problem is that human eggs for research are in short supply and to obtain them women have to undergo surgery.
That is why scientists want to use cows' eggs as a substitute.
They would insert human DNA into a cow's egg which has had its genetic material removed, and then create an embryo by the same technique that produced Dolly the Sheep.
The resulting embryo would be 99.9% human; the only bovine element would be DNA outside the nucleus of the cell.
It would, though, technically be a chimera - a mixing of two distinct species into one.
The aim would be to extract stem cells from the embryo when it is six days old, before destroying it.
The quality and the viability of stem cells would then be checked to see if the technique had worked.
The scientists also plan to examine the way the cells are reprogrammed after fusion to see if there are useful processes they could replicate in the laboratory.
Lead researcher Dr Lyle Armstrong said: "If we can learn from the egg cell how to make embryonic stem cells without having to use an animal egg at all then some day we may be able to cure diseases such as Parkinson's disease, or better still some of the age-related diseases which are creating such a burden on society."
Dr Stephen Minger, from King's College London, said: "The current state of the technology is such that literally hundreds of human ooctyes (eggs) from young women will be required to generate a single human embryonic stem cell line.
"Therefore we consider it more appropriate to use non-human oocytes from livestock as a surrogate.
"We feel that the development of disease-specific human embryonic stem cell lines from individuals suffering from genetic forms of neurodegenerative disorders will stimulate both basic research and the development of new medicines to treat these horrific brain diseases."
Professor Robin Lovell-Badge, head of developmental genetics, National Institute for Medical Research, said: "This is a very rational step: to learn what you can using animal eggs, which are readily obtainable, before moving on to valuable human eggs when or if this becomes necessary."
But some will argue the end does not justify the means.
Calum MacKellar, from the Scottish Council on Human Bioethics, said the research undermined the distinction between animals and humans.
He said: "In the history of humankind, animals and human species have been separated.
"In this kind of procedure, you are mixing at a very intimate level animal eggs and human chromosomes, and you may begin to undermine the whole distinction between humans and animals.
"If that happens, it might also undermine human dignity and human rights."