FLASHPOINTS IN THE EMBRYO RESEARCH DEBATE.
AThere are four main issues causing friction. The Bill would give the green light to research using hybrid or 'admix' embryos, where the nuclei of a human cell is inserted into an animal egg.
The resulting embryos would be kept for up to 14 days in order to harvest stem cells, which are crucial for many cutting edge medical treatments.
The legislation also backs the idea of "saviour siblings" - meaning parents would be allowed to select children whose genetic material could help sick relatives.
Thirdly, the requirement for IVF clinics to consider the "need for a father" when handing out treatment could be removed - giving lesbians
Amendments have also been tabled which would tighten abortion laws, by reducing the upper limit from 24 weeks to 22, 20 or 16.
QWhy is the Bill so important to science?
ASupporters believe that the creation of human-animal embryos could help scientists find cures for diseases including multiple sclerosis, motor neurone and Alzheimer's. Currently, the availability of stem cells is limited due to shortages in human eggs.
QWho is supporting the Bill's aims?
AGordon Brown has passionately championed the need to facilitate stem cell research, but bowed to concern among ministers by allowing a free vote on the controversial elements of the Bill. Gay rights protesters and many scientists back the proposed embryology changes. The British Medical Association has insisted there is no evidence that the upper abortion limit needs changing.
QWho is against the Bill?
AReligious leaders have attacked many elements of the Bill, and Catholic Cabinet ministers Ruth Kelly, Des Browne and Paul Murphy are expected to oppose key sections.
The Catholic church objects to the use of animal-human embryos, known as "chimeras", because it believes a fertilised embryo should be regarded as human, and should not be destroyed or terminated.
Catholics and Church of England figures such as the Archbishop of York criticise the reforms of IVF rules, claiming they undermine the role of fathers and of the family.
Demands for reducing the upper abortion limit are also strongest among religious groups.