FORTY YEARS ago, a successful bone-marrow transplant between two siblings marked the first time groups of inherited immune system disorders in babies no longer automatically led to a new life being cut short. Bone marrow transplants, or stem cell transplants, involve abnormal stem cells from a person's bone marrow being destroyed and replaced with healthy stem cells from that person or from a donor. The procedure, once revolutionary, is now used daily worldwide and saves countless lives. Yet in medical terms it is relatively new, with advances made year-on-year as new research yields new success in the fight against diseases such as leukaemia.

Since the early 1960s, when two Canadian scientists, Ernest McCulloch and James Till, first began looking at the stem cells, medical research has never doubted their potential to make a hugely positive contribution to the treatment of human diseases including cancer, Parkinson's disease, spinal cord injuries and many others. Certainties don't exist in clinical research, as the long fight to discover cures and new therapies for HIV and Aids has shown. But it is a certainty that if stem-cell research simply halts, or backs off from new approaches, then nothing of value will be discovered.

Writing in this newspaper, and speaking later today from the pulpit of a Roman Catholic church in Edinburgh, Cardinal Keith O'Brien states that claim after claim has been made for research that involves the use of stem cells taken from human embryos, and that after a decade of promised cures and treatments, not one new treatment or therapy has arrived. Like any prominent church leader, the cardinal is entitled to his opinion, an opinion that in the case of Roman Catholics in Scotland is expected to be followed without question as part of the church's wider teaching.

In the case of Catholic MPs, the archbishop expects to see them vote against the government's Human Fertilisation and Embryology Bill which allows, on the archbishop's interpretation of the science, "the creation of hybrid human-animal embryos" a procedure which involves a "monstrous attack on human rights, human dignity and human life".

The cardinal's moral outrage includes a comparison with the experiments of Dr Frankenstein and the lack of scientific restraint that led to the creation of the atom bomb by Dr Robert Oppenheimer. An extension of abortion laws, legalised raiding of a dead person's tissue, legalised creation of babies whose sole purpose is to provide spare parts, will all follow if the new bill is passed, he claims.

The moral and theological pressure being put on Catholics by leading clergy such as Cardinal O'Brien is high-octane and forceful, and its deliberately emotional content is no accident.

The right of Cardinal O'Brien to preach to the Catholic faithful is entirely justified. But there is no justification for Cardinal O'Brien to try to impose his church's views, politically and morally, on those outside the Catholic faith.

At best, he has simply failed to understand the complexity of human embryonic stem-cell research. As Dr Stephen Minger points out today in this newspaper, there is no hybrid mix of human and animal cells, no "Frankenstein" science, no fusion of animal and human DNA. The transplantation of skin cells from people suffering from major neurological disorders into an animal egg that has had its own DNA removed is a procedure difficult to comprehend, even for those with a scientific background. But to wrap clinical complexity up in a misinterpreted, misplaced emotional package of theological and moral outrage does little to help us understand what might lie ahead in this chapter of stem-cell biology.

The terminology of medical research is often difficult to decode. Hybridised animal-human embryos, sometimes called "chimera" embryos, involve taking the DNA from human skin cells and merging it with the cytoplasm, the non-nucleus part of the cell, of an unfertilised animal egg. The embryonic stem cells which can then be created provide the foundation to test new treatments for diseases such as Parkinson's. There is no evidence to suggest chimera embryos could develop normally and it would remain illegal to allow them to develop beyond 14 days.

Yet the mere mention of the word "embryo" has turned what should have been seen as a valued piece of new medical legislation into an unnecessary moral combat zone with the Catholic Church on one side, and seemingly irresponsible scientists and compliant politicians on the other.

Adding fuel to what has become a controversial moral issue, rather than a debate over the merits of new clinical research, is the prime minister, and yet another case where a vacuum of parliamentary authority is leading to chaos.

By allowing a free vote to all Labour MPs, Gordon Brown would have allowed Catholic MPs to either take the advice of people such as Cardinal O'Brien or perhaps do a bit of their own research and decide if Frankenstein really was at the centre of stem-cell advances. But by not making it clear if a three-line whip is going to be imposed, the PM has created a heated forum where the views of Cardinal O'Brien and others are given a public currency rather than being confined to the teachings within their own church.

Brown should quickly make it clear there will be no three-line whip, and that Labour MPs will be allowed to vote as their conscience directs. If the bill does not pass, this field of clinical research and the benefits it may bring, will not be lost. It will be conducted in another country - and when the medical advances do come, which one of us will turn them away or deny them to our children?