(CNSNews.com) - After weeks of rumbling rebellion over his refusal to allow Labor lawmakers a free vote on a highly controversial human embryo research bill, British Prime Minister Gordon Brown backtracked on Tuesday, but he won little praise for the concession.

Under fire from Catholic bishops and facing opposition from some cabinet colleagues, Brown said that during various stages of the legislative process in the coming months, his party's MPs would be allowed a free vote on specific areas in the human fertilization and embryology bill that have proven the most contentious.

But when the bill is put to a vote after any amendments are incorporated, all Labor lawmakers will be expected to vote the party line -- whether they consider the final version acceptable or not.

Announcing the move, Brown said the bill itself could not be the subject of a free vote because it covered "so many other changes I believe are necessary as part of building up the research framework of our country and ... creating the right ethical framework for the development of embryo research."

Some opponents of the bill said the compromise did not go far enough, and the prime minister also came under fresh attack from the opposition Conservative Party, whose MPs are being allowed to vote according to conscience.

Of the parts of the bill that have caused most alarm, the most contentious is one that would allow the creation part-animal, part-human embryos (by injecting human cells into a rabbit or cow egg that has had its own DNA removed, or vice versa). The aim is to provide scientists, now hampered by a shortage of human eggs, with a larger supply of embryonic stem cells (ESCs) for research purposes. Proponents of ESC research say the cells - "building-blocks" of all tissue, with the ability to grow into nerve, muscle or other cells - could someday provide treatments or cures for debilitating diseases.

Also drawing opposition is a provision that would allow parents to create so-called "savior siblings" -- to genetically pre-select an embryo during IVF treatment so that the resulting child will be a suitable future donor of tissue or bone marrow for an existing brother or sister with a rare disease.

Another provision will prevent fertility clinics from refusing IVF treatment to single women or lesbians, by removing a current requirement for clinics to consider a child's need for a father before approving patients.

Pro-lifers say the legislation will lead to the further abuse and destruction of "embryonic human lives," and a senior Catholic leader described it as an attack on human life and dignity, calling the idea of mixing animal and human genes "not just evil" but "crazy."

In a letter to lawmakers on Tuesday, Brown defended the human-animal hybrid embryo proposal, saying it will be illegal to keep them for more than 14 days or to implant them into a human or animal womb to gestate. He also pointed out that the DNA of stem cells harvested from the embryos will be "overwhelmingly human."

Brown said he believed the measures contained in the bill were "vital to the progression of stem cell research."

Among those who welcomed Brown's decision were leading scientists who expressed hope it would take some of the political heat out of the debate.

"The potential medical benefits of this research to millions of people with terrible incurable diseases are simply too important for us to allow this debate to descend into a political row," said Dr. Stephen Minger, head of the Stem Cell Biology Laboratory at King's College London.

But the Conservative Party's spokesman on health, Andrew Lansley, said Brown was still "dithering" on the issue. If the bill was to enjoy public confidence, "MPs must be allowed to exercise their judgment" by having a free vote on the entire bill, he said.

Members of the third-largest party in the House of Commons, the Liberal Democrats, will be allowed a conscience vote on the more controversial aspects of the bill.

In its reaction, the Catholic Church in Scotland noted opponents would only be allowed a free vote during intermediate stages of the legislative process, but in the end would be compelled to back the bill.

As such, said spokesman John Deighan, it "still leaves Labor MPs in a position where they may have to vote for something which they believe to be intrinsically wrong."

"MPs shouldn't vote for animal-human hybrids because they are scientifically unnecessary," he said. "We know that adult stem cell research has been able to provide over 70 useable therapies, which the Catholic Church supports."

Cardinal Cormac Murphy-O'Connor, head of the Catholic Church in England and Wales, welcomed the prime minister's compromise, but also reiterated a church call for the establishment of a national bioethics committee "to act as the focal point for discussion and advice on these matters."

O'Connor pointed to what he called "exciting developments in research using adult stem cells that do not involve the deliberate creation and destruction of human life, or the mixing of human and animal life."

"Adult" stem cells originate from non-embryonic sources such as bone marrow, placentas, umbilical cords and nasal passages. Scientists differ over their potential compared to embryonic cells, while supporters contend that they have demonstrated their therapeutic worth in the treatment of numerous medical conditions.

Late last year, two groups of scientific researchers reported that they had succeeded in turning human adult skin cells into cells that behave like ESCs, thus sidestepping the ethical concerns surrounding the destruction of embryos for their cells.

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