Prime Minster Gordon Brown has written to Labour MPs about the embryology bill, after warnings that some of his party would rebel and vote against the controversial legislation.
Gordon Brown has faced calls to give Labour MPs a free vote
Here is the full text of his letter:
I wanted to write to you to set out my reasons for supporting all the measures in the Human Fertilisation and Embryology Bill, and to clarify how we propose to handle it in Parliament.
The Human Fertilisation and Embryology Bill will update the legal framework which governs assisted reproduction and embryo research, bringing this into line with recent scientific developments and equalities legislation.
It will reform the regulatory framework in which embryo research is conducted, ensure that all human embryos outside the body are subject to the best regulation, create a ban on sex selection of offspring for non-medical reasons, and update the rules around entitlements to IVF.
Whilst many of these measures will not be controversial, the legislation will also provide the legal framework to support the continued development of stem cell research and clearly there are differing views on all sides of the house on this question.
Stem cell research is important because it makes it possible for this generation to contemplate new and effective treatments and cures for diseases that have afflicted mankind from time immemorial - from leukaemia and Alzheimer's to conditions affecting every family such as cancer and heart disease.
It was only in 1998 that James Thomson, a scientist at the University of Wisconsin, successfully removed stem cells from spare embryos at fertility clinics in Madison.
His discovery established the world's first human embryonic stem cell line and launched the science of stem cell research into the scientific mainstream.
Since then, work, particularly in the US and the UK, has demonstrated that embryonic stem cells have the potential to be used by medical science to generate replacement cells for a broad array of tissues and organs such as the heart, liver, pancreas and the nervous system.
The UK is at the forefront of this research and responsible for much of the worldwide progress.
With adult stem cells already being used in treatments for conditions including leukaemia, severe combined immunodeficiency, and heart disease, scientists are already close to the breakthroughs that will allow embryonic stem cells to be used to treat a much wider range of conditions.
Medical researchers now believe that stem cell therapy has the potential to change dramatically the treatment of many other human afflictions: including not only Parkinson's disease and Alzheimer's but perhaps also cancer, spinal cord injuries and muscle damage.
Stem cell research has always been controversial and I appreciate that there have always been those who have raised objections on religious or moral grounds as well of those of religious faith who have supported this type of work.
I take these concerns seriously and respect the views of all those with religious convictions which they see as precluding this type of research.
I also see the profound opportunity in lives and quality of life which will be saved if we are able to answer objections and pursue the treatments and cures which will be available to us through this strand of medicine.
That is why we have - patiently and with full regard for religious concerns - sought to introduce progressively clear laws which permit the use of stem cells but which do so within a clear, managed legal framework, subject to the strictest supervision.
It is this approach which has ensured that Britain's support for research into stem cells has helped advance this science.
It is not only in our own interest, but also the global interest that Britain should continue to play this role.
One major problem which stem cell researchers around the world now face is that embryonic stem cells are in short supply, and this is now limiting the pace of progress towards future cures and treatments which will save many lives.
That is why it is argued that the safest way to support our science is to make use of animal eggs from which animal genetic material has been extracted, and which can then be made compatible for research on human diseases by the inclusion of a human cell nucleus.
"Human admixed embryos", as they are known, are made by inserting human genetic material in the nucleus of an animal egg cell.
If the embryo survives for a few days, stem cells may be collected and grown in culture. These cells contain overwhelmingly human DNA.
Permitting the use of these admixed embryos - which, it should again be emphasised, contain only minuscule amounts of animal DNA - will bring to an end the critical limiting factor in stem cell research: the limited availability of human eggs from which to create embryos.
The 1990 act is silent on the rules surrounding the use of these admixed embryos.
Research on genetic conditions such as Alzheimer's has progressed on the basis of this approach in London and Newcastle with the support of the regulator, the HFEA, but both teams face uncertainty because of the lack of clarity in the rules.
The purpose of this 2008 Bill is to provide this clarity and a clear framework which supports stem cell research but provides boundaries.
Admixed embryos will be allowed only where they are used for clear scientific purposes and it will be illegal to keep them beyond 14 days. It will also be illegal to implant them into women or into any animals.
And so, in the next couple of months, legislation to create this framework and thus allow further progress in vital research will come before the House of Commons.
The legislation as a whole will be government business at the second and third readings.
However, respect for the religious beliefs of those who have fundamental concerns means we will have a free vote on the three new ethical issues on which the House has not previously taken a view: on whether admixed embryos are permitted within strict constraints; on the question of permitting 'saviour siblings' (who could, for example, donate blood) in the context of rare genetic conditions; and on changing the IVF requirements to require the need for supportive parenting to be taken into account.
I would like to take this opportunity to tell colleagues now that I will be voting in favour of these measures, including that to permit the use of admixed embryos which are, in my view, vital to the progression of stem cell research.
However, I fully respect the views of those who have specific religious objections.
I am also reassured that leading doctors and scientists do not believe they should operate in the absence of clear rules, but they believe that theirs is an inherently moral endeavour, that they can save and improve the lives of thousands and over time millions of people, and that they can combine this work with a deep commitment to the highest ethical standards and a sincere respect for religious beliefs.