LONDON (AP) - British lawmakers have passed a measure that would relax the rules limiting medical research on human embryos, rejecting opponents' claims that the move is a step toward permitting the cloning of human beings.

The measure passed Tuesday has not yet completed the legislative process. But if it becomes law, it effectively would enable scientists to clone embryos and keep them alive for up to 14 days to extract so-called stem cells - the unprogrammed master cells found in early stage embryos that can turn into nearly every cell type in the body. Cell-based treatments are expected to open a new chapter of medicine, raising the hope of prevention or cure for ailments from Parkinson's disease to diabetes.

The amendment to the 1990 Human Fertilization and Embryology Act passed 366 to 174 in the House of Commons. Before the vote, government officials downplayed the connection between the change and concerns about human cloning.

``Parliament is not being asked to cross the Rubicon today,'' said Public Health Minister Yvette Cooper. ``Human reproductive cloning is illegal and must stay illegal.''

The legislation now goes to the unelected upper chamber of Parliament, the House of Lords, which can delay legislation but can be overruled by the government.

Opponents of the move, including anti-abortion and religious groups, said lawmakers had failed to consider the ethical implications of the decision.

``Cloning, even for therapeutic purposes, is a new form of human reproduction with massive moral implications,'' said Archbishop of Westminster Cormac Murphy O'Connor, head of the Roman Catholic church in England and Wales.

``As a society, do we really want to make this dangerous leap without much more thought and reflection?'' he asked. ``I hope that the House of Lords will reject these proposals.''

Research and patient groups, though, applauded the decision.

``In future, stem cell research may lead to the development of new treatments for heart diseases and may even allow hearts or other tissue to be grown for transplantation,'' said Sir Charles George, medical director of the British Heart Foundation. ``We are a long way from reaching these goals, which is why stem cell research must be fully explored.''

The existing law permits research using donated human embryos only for strictly limited purposes, including studies on infertility and the detection of genetic abnormalities. The change would extend the law so that early stage embryos could also be used for research into diseases that emerge in adulthood.

The Human Fertilization and Embryology Authority, which polices embryo research, has said it would not license any research aimed at producing cloned babies. But it has promised to consider cloning applications for specific types of research on embryos up to 14 days old. Stem-cell research would inevitably involve cloning of embryos, because the goal is to treat patients with perfectly matching tissue.

An embryo is essentially a ball of stem cells that evolves into a fetus when the stem cells start specializing to create a nervous system, spine and other features - at about 14 days.

Scientists hope that by extracting the stem cells from the embryo when it is three or four days old, their growth can be directed in a lab to become any desired cell or tissue type for transplant.

The hope is that one day it will be possible to grow neurons to replace nerve cells in a brain killed by Parkinson's disease, skin to repair burns and pancreatic cells to produce insulin for diabetics.

Scientists would create a clone of a sick patient by removing the nucleus of a donor egg and replacing it with that of a cell from the patient. The egg would be induced to divide and start growing into an embryo. The cloned cells would be genetically identical to the patient's and therefore theoretically overcome problems of transplant rejection, which happens because the immune system fights foreign tissue.