As parliament prepares for a two-day debate on the Human Fertilisation and Embryology Bill, the Prime Minister staged an impassioned defence of embryo research on Saturday.
Mr Brown said developing "hybrid" techniques was an "inherently moral endeavour".
But the Catholic Church of Scotland last night criticised his comments as a contradiction in terms.
The Prime Minister, whose son Fraser suffers from cystic fibrosis, a condition that could benefit from the research, said while he "deeply respected" the views of his religious colleagues, he was clear that the use of so-called admixed embryos had to be given the go-ahead. "I have deep respect for those who do not agree with some of the provisions in the bill because of religious conviction," he said.
"But I believe that we owe it to ourselves and future generations to introduce these measures, and in particular, to give our unequivocal backing within the right framework of rules and standards, to stem-cell research."
He added: "Let me be clear: if we want to sustain stem-cell research and bring new cures and treatments to millions of people, I believe admixed embryos are necessary. The question for me is not whether they should exist, but how their use should be controlled."
Mr Brown sent a clear signal to his MPs and the country on where he stood, although many parts of the bill will be subjected to a free vote.
But his view was dismissed as "strange" by the Catholic Church of Scotland, which has also questioned why millions of pounds of taxpayers' money is going on embryology research.
Peter Kearney, a spokesman for the Church said: "There is nothing moral about the treatment of human life as a commodity which is what this bill does.
"To say that animal-human hybrids are moral is a complete inversion of the meaning of the word."
The Church also criticised Mr Brown for trying to make the issue one of "science versus religion".
Mr Kearney argued that it was not that at all, but rather "good science versus bad science".
While stem cells did not lead to cures, they had already led to therapies, while the science around human embryos was still unproven, he said.
In response, the Catholic Church of Scotland last night also launched a 25,000 grant to help adult stem-cell research.
Opponents of the bill, however, are resigned to seeing the legislation passed into law.
Scientists want to insert the nuclei of human cells into animal eggs, creating hybrid embryos which are allowed to grow for a few days.
Stem cells can then be harvested and used to create brain, skin, heart and other tissue for treating diseases – before the embryos are destroyed.
MPs will have a free vote on the hybrid embryos today as well as making a decision on "saviour siblings" – whose genetic material could help sick relatives. Tomorrow will also see a crunch vote on reducing the time limit for abortion and allowing lesbians equal access to IVF.
Government believes time has come to update 20-year-old laws on research
What is an admix embryo?
A hybrid or admix embryo is a mixture of both human and animal tissue. It is created when DNA from human cells, such as skin cells, is transferred into animal eggs that have had almost all of their genetic information removed, or vice versa. The resulting cytoplasmic embryos – or admixed embryos – are more than 99 per cent human, with a small animal component.
What are the different kind of hybrids?
Chimeras, which are formed by merging human and animal embryos; hybrids created by fertilising a human egg with animal sperm, or vice versa; and cybrids which are formed when human DNA is inserted into an animal egg from which the nucleus has been removed.
Will these be allowed to grow into full size animal-human hybrids?
No. The embryo would be grown in the lab for a few days, then harvested for stem cells. The bill seeks to regulate embryonic research which is already happening. Scientists have to seek licences to show that it is for medical purposes before being granted permission to conduct research on "human admixed embryos".
Why is it so important to science?
Supporters of the bill believe that the creation of human-animal embryos could help scientists find cures for diseases including multiple sclerosis, motor neurone disease and Alzheimer's. Currently, the availability of stem cells is limited due to shortages in human eggs.
Medical research and technology have progressed hugely since the Human Embryology Act of 1990, and experts say the law needs to be updated to reflect those changes.
Why is the law being updated now?
THE laws governing fertility treatment and embryo research are nearly two decades old. The Human Fertilisation and Embryology Bill also reforms IVF rules, which at present can discriminate against lesbians and single women.
Why are Labour MPs not allowed a free vote on the entire Bill?
Although matters of fertility are usually decided as a conscience vote, the government was worried that opponents of research using human embryos would scupper the bill in its early phases. There are free votes on some of the contentious parts, however.
Who backs it?
Gordon Brown has championed the need to facilitate stem cell research, but bowed to concern among senior ministers by allowing a free vote on the controversial elements.
Who is against it?
Religious 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.
Days of heated debate as MPs examine consciences
TWO days of heated debate in the House of Commons will kick off this afternoon, with a discussion of human-animal hybrid embryos.
The Human Fertilisation and Embryology Bill is at committee stage in the House of Commons.
It has already passed its first two hurdles in the Commons.
Today, MPs will vote on two contentious areas: the creation of human-animal hybrids (so called admix embryos) and the issue of "saviour siblings".
Edward Leigh, a senior Conservative MP, has tabled an amendment to ban all such experiments but this is unlikely to pass.
However, a second amendment from Andrew Lansley, the shadow health secretary, wants to outlaw pure hybrids made by fertilising human eggs with animal sperm. Although there are no plans by scientists to do this, this stands more chance of success.
The second free vote tonight will be on whether to allow the creation of stem cells in order to help a sick sibling.
Mr Lansley has also tabled an amendment on this, which would restrict help to cells, rather than entire organs, being transplanted to siblings.
Arguably, the more contentious parts of the bill are being debated tomorrow.
The first three-hour debate will be on a child's right to a father. Equality campaigners have argued that the way the law is currently worded allows discrimination against lesbians seeking IVF treatment.
Before providing fertility treatment, at the moment clinics must consider a child's "need for a father". The government is proposing to replace this with "need for supportive parenting".
Opponents of this argue that few lesbians have been denied treatment because their partner is not male.
Iain Duncan Smith, the former Conservative leader, has tabled an amendment calling for clinics to consider the right for a "mother and a father". Mr Lansley has also tabled an amendment to this, which would call for a father or "male role model". Religious campaigners believe it is on this point that they stand the most chance of success.
The most contentious issue is left until tomorrow night when MPs will have the first chance in 30 years to reduce the term limit for abortion from 24 weeks.