LONDON , England ( CNN ) -- Opponents of controversial plans to use hybrid human-animal embryos for research spoke out Tuesday , calling the practice unnecessary , unnatural , and reprehensible a day after British lawmakers voted to allow it . Advocates of hybrid cells say animal eggs from which a nucleus has been removed are simply `` empty shells . '' The British parliament debated the issue Monday as part of its discussion of the Human Fertilization and Embryology Bill , which will update legislation on reproduction and embryos . `` Crossing the species barrier in this way is deeply , deeply reprehensible , undesirable , '' said Josephine Quintavalle , a bioethicist who founded Comment on Reproductive Ethics ( CORE ) . The research involves emptying an animal egg and filling it with human cells . The resulting embryo is allowed to develop for 14 days -- during which time scientists harvest the stem cells -- before being destroyed . Scientists hope working with those stem cells will lead to treatments for serious conditions like motor neuron disease , Parkinson 's , and Alzheimer 's , and they say using hybrids overcomes the shortage of human embryos . `` The use of animal eggs will provide a valuable resource to embryo research scientists , '' argued Public Health Minister Dawn Primarolo . Lawmakers were allowed to vote according to conscience and the legislation passed comfortably , though not without opposition . `` If an embryo could talk , perhaps it would echo what Mary Shelley did say in 'Frankenstein ' : ' I , the miserable and the abandoned , an abortion to be spurned out and kicked and trampled on , ' '' Conservative Party lawmaker Edward Leigh told parliament . Supporters of the research dispute the numerous `` Frankenstein '' references and say it does not create monsters . Stephen Minger , director of the stem cell biology lab at King 's College in London , said the practice begins with the removal of an animal egg 's nucleus , which contains all of the chromosomes , thereby stripping the egg of its `` species identity . '' `` It 's an empty shell , '' Minger said . `` By putting a human cell -- not just a nucleus , but in our case an intact human cell -- into the egg , you confer a human genetic identity onto that . '' The approved bill creates a legal framework for the scientific research on hybrid embryos . British Prime Minister Gordon Brown feels so strongly about the benefits of the research that he wrote a column Sunday in The Observer newspaper calling it an `` inherently moral endeavor , '' but ethics groups and others disagree . `` I think we all know that we do n't reproduce with animals , '' Quintavalle said . `` Humans do not reproduce with animals . Whether it 's done in the laboratory or not does n't make it right . '' Quintavalle urged the government and the scientific community to wait until a new method of research emerges which does not mix animal and human genetic material . Human Genetics Alert , a secular independent watchdog , said it found defects among existing hybrid embryos which raise doubts about whether mixed embryos can produce useful stem cells . `` I 'm very , very unimpressed with the scientific case for doing that , '' said David King , a former molecular biologist who now heads the HGA . `` The science is so weak and the ethical concerns are so significant , I think you have to weigh that . '' The scientific arguments in favor of the research have been `` overhyped , '' King said , and offer no hope of a cure for those suffering with genetic disorders . `` Very little , I think , will come out of it and I think hopes are being raised that will be cruelly disappointed , '' King said . The Human Fertilization and Embryology Bill was drawn up in 1990 , when science and government were unaware that current hybrid-embryo developments were possible , a spokeswoman for the Department of Health told CNN . The original bill did allow the mixing of human and animal eggs , but only for the purpose of testing the fertility of human sperm , said the spokeswoman , who declined to be named in line with policy . The practice was known as the `` hamster test '' because hamster eggs were used . The revised bill now allows hybrid embryos for further research but specifies they may not be placed in a woman or an animal , the spokeswoman said . Also , those using the embryos must prove that the use is for research , and that use of an embryo is necessary , she said . Creating hybrid embryos is not the only way to develop useful stem cells , said Minger . A new method called pluripotency , developed in December , allows scientists to turn adult cells into something resembling an embryonic stem cell , he said . Pluripotency , however , requires extensive genetic manipulation of the cells and yields few that are of use to research , Minger said . `` We will pursue that ( research ) but it 's just too early to know which is the best approach , '' Minger said . `` Most rational scientists would say we need to proceed down as many tracks as possible because we just have no idea what 's going to lead to the best benefit . '' CNN 's Phil Black contributed to this report .
UK lawmakers approve bill allowing use of hybrid embryos in stem cell research . Opponents describe controversial technique as `` deeply , deeply reprehensible '' Advocates say use of animal embryos helps overcome shortage of human embryos . Scientists say stem cells have potential to cure conditions such as Parkinson 's .