A bill is going through Parliament which would allow scientists to create human-animal embryos for research.
Early embryos yield stem cells
Researchers say the work is needed to advance the understanding of complex diseases such as Parkinson's, Alzheimer's and Motor Neurone Disease.
But critics say it involves the needless destruction of human life, and is fraught with moral difficulties.
What is a hybrid?
A hybrid embryo is a mixture of both human and animal tissue.
The experiments that some British scientists want to conduct involve transferring nuclei containing DNA from human cells, such as skin cells, into animal eggs that have had almost all of their genetic information removed.
The resulting cytoplasmic embryos - known as admixed embryos - are more than 99% human, with a small animal component, making up around 0.1%.
The embryo would be grown in the lab for a few days, then harvested for stem cells - immature cells that can become many types of tissue.
There are other types of hybrid embryo, although research into these alternatives is not thought to be at such an advanced stage. They include:
True chimeras: The addition of one cell from an animal embryo to a human embryo. The subsequent embryo is made up of cells that are either wholly human or wholly animal.
Transgenic human embryos: A human embryo which has been genetically modified to contain a small amount of animal DNA, for instance one or more animal genes. Each cell would have the usual complement of around 20 000 human genes, and a couple of animal ones.
True hybrid: Fertilisation of a human egg by an animal sperm or vice versa. The resulting embryo would be approx 50% human and 50% animal.
Why use animal eggs?
The creation of hybrid human-animal embryos was first suggested as a way of addressing the shortage of human eggs available for research.
Also, experts say using human-animal mixes rather than human eggs to get the stem cells makes sense because the process is less cumbersome and yields better results.
How could this help find disease cures?
Scientists say they can use embryonic stem cells to study different disease processes.
For example, they could take genetic material from a person with Parkinson's disease and put it into an empty animal egg to make stem cells that will carry the same genetic defects that cause Parkinson's.
Stem cells also have the potential to grow into different tissues, so in the future it might be possible to transplant cells cloned from individual patients to cure diseases.
What are the concerns about the work?
Opponents say this is tampering with nature and is unethical.
Critics say they are repulsed by the idea and there must be no creation of an animal-human hybrid.
It is already illegal to implant human-animal embryos in the womb or bring them to term.
One of the scientists applying to do hybrid work, Professor Chris Shaw from Kings College London, stressed: "We think there is nothing illegal, immoral or unethical about this.
"While we understand the concerns, we think they are largely founded on misinformation.
"People think we are generating some sort of hybrid animal. This is just cells, just for science. No animal is ever going to be created."
What does the law say?
Although ministers felt the Human Fertilisation and Embryology Act 1990 needed updating as science had moved on significantly, they were initially cool on the idea of creating human-animal embryos.
However, they bowed to pressure from scientists, who argued a ban would hamper medical research.
In addition, the House of Commons Science and Technology Committee carried out an inquiry
into the proposals and concluded that the creation of hybrid embryos was necessary for research and that there should not be a complete ban.
A draft bill published in May 2007 allows for the creation of human embryos that have been physically mixed with one or more animal cells.
Will the bill become law?
Opponents are determined to block the proposals. Several leading members of the Catholic Church have described them as immoral.
In his Easter sermon, Cardinal Keith O'Brien, leader of the church in Scotland, described the legislation as a "monstrous attack on human rights, human dignity and human life".
A significant number of MPs, including several Catholic members of the Cabinet, are also reported to have serious concerns about the legislation, leading to speculation of high profile resignations.
Prime Minister Gordon Brown has promised that MPs will be given a free vote on some of the more controversial aspects of the bill, including human-animal embryos.
Some 200 medical charities have urged MPs to support legislation allowing the creation of animal-human embryos.
And Labour peer and fertility expert Lord Winston has accused the church of misleading the public over the issue.
Are there other controversial elements to the draft bill?
Yes. The draft bill approves the creation of so-called Saviour siblings. This would allow doctors to select an embryo for IVF which could not only create a new child, but also tissue which may be able to treat an existing sick sibling.
The bill would also end the need for IVF clinics to consider the need for a "father figure" when deciding whether or not to offer treatment.
Will the scientists be able to proceed with their work?
Two licences to create one type of hybrid embryo have already been granted by the Human Fertilisation and Embryology Authority and a team at Newcastle University has successfully produced them.
But the licensing body has also stressed that each application must be considered closely on its own merits.
If future government legislation differs from the HFEA line, then the regulator will be forced to rethink.