LONDON — British plans to allow scientists to use hybrid animal-human embryos for stem-cell research won final approval from lawmakers Wednesday in a sweeping overhaul of sensitive science laws.

The House of Commons also clarified laws that allow the screening of embryos to produce babies with suitable bone marrow or other material for transplant to sick siblings.

It was the first review of embryo science in Britain in almost 20 years.

The legislators voted 355 to 129 to authorize the proposals after months of sometimes bitter debate that has pitted Prime Minister Gordon Brown's government and scientists against religious leaders, anti-abortion campaigners and others anxious about medical advances.

Brown says he believes scientists seeking to use mixed animal-human embryos for stem-cell research into diseases such as Parkinson's will help improve — and save — millions of lives.

Decisions by Britain's Human Fertilization and Embryology Authority, an independent body which regulates fertility and embryo research in the U.K., to allow the practice have previously been vulnerable to challenges in court.

While Britain has been seen as a world leader in stem cell and cloning research, similar work to create human embryos from animal eggs is also being conducted in China and the United States.

British lawmakers had already endorsed individual proposals, but Wednesday's vote involved the complete draft bill.

"One in seven couples need help with fertility treatment, 350,000 people live with Alzheimer's, every week there are five children born and three young people die from cystic fibrosis — all issues that this bill addresses," Health Minister Dawn Primarolo told lawmakers, opening a debate on the draft laws.

Britain's government opted not to allow legislators to use the debate to consider the country's abortion laws — last drafted in 1990 — frustrating hopes of both anti-abortion lawmakers and those seeking to liberalize current regulations.

Ministers said lawmakers needed to focus on important revisions to rules governing stem cell research and other scientific advances, rather than examine the emotive issue of abortion — which isn't covered by the draft laws.

Brown is a strong advocate of stem cell science and has said Britain owes it to future generations to support the research. Opponents warn an easing of laws on creating embryos could lead to the genetic engineering of human beings.

The process involves injecting an empty cow or rabbit egg with human DNA. A burst of electricity is then used to trick the egg into dividing regularly, so that it becomes a very early embryo, from which stem cells can hopefully be extracted.