The head of the Catholic Church in England and Wales said yesterday he was "gravely concerned" by the Commons decision to approve a Bill which allows the cloning of human cells for medical research.
The Archbishop of Westminster, Dr Cormac Murphy O'Connor, appealed to the House of Lords to oppose the decision.
Scientists say the proposal will allow huge advances in the treatment of disease, but opponents believe it raises serious ethical questions.
"Cloning, even for therapeutic purposes, is a new form of human reproduction with massive moral implications," Dr Murphy O'Connor said in a statement. "As a society, do we really want to make this dangerous leap without much thought and reflection?"
MPs voted on Tuesday night to relax the regulations on cloning and the use of human embryos for medical research. It will not become law until it has been approved by the Lords, who are expected to vote on the issue in the next few days.
According to scientists who back the relaxation of the rules, it would allow them to make great strides in the treatment of burns, spinal injuries, brain disease and organ failure. Cells from a patient could eventually be cloned and cultivated in a laboratory into purpose-built tissue which is then transplanted into a patient to replace the damaged tissue.
The advantage of the technique is that the transplanted tissue is not likely to be rejected, as often happens with donated organs, and it will be more readily available than it is now.
The Archbishop was joined in his criticism by Prof John Scarisbrick, chairman of the anti-abortion charity Life. "What parliament has done, by the back door, is to permit the creation of an entirely new kind of human being, a human being who is asexually generated with no mother or father", he said. "Society should tremble at the prospect of doing this. We haven't even begun to consider the ethical issues."
The Public Health Minister, Ms Yvette Cooper, reassured the Commons on Tuesday that human reproductive cloning was illegal and, "under these regulations, it will stay illegal". But she said research into cloning could hold "the key to healing within the human body", giving hope to those suffering from degenerative diseases, and also to cancer and heart disease victims.