Ann Widdecombe, an opposition Conservative lawmaker, said there is no proof that hybrid embryo research could help treat diseases that currently have no cure.
"There is no evidence at all that it will save millions of lives," Widdecombe told Britain's GMTV.
Robin Lovell-Badge, a stem cell biologist at the U.K.'s National Institute for Medical Research, said that greater understanding of genetic diseases at the cellular level could speed the development of treatments.
"We have to be careful not to overhype it, because we can't promise anything will work," he said.
"But if it does work then there will be a lot more understanding. More understanding is crucial to developing new treatments."
Still other opponents warn that an easing of laws on creating hybrid embryos could lead to genetic engineering of human beings.
Human Genetics Alert, a science watchdog group that opposes the changes, says the laws could lead to the creation of so-called designer babies.
"Once we start down the road to human genetic modification, it will be very difficult to turn back," the group warns in a briefing paper for lawmakers.
Savior Siblings
The newly changed law is part of the first major review of embryo science in the U.K. for almost 20 years.
Britain's Human Fertilization and Embryology law, which regulates all stem cell and embryology research, was drafted in 1990.
Debates taking place in the House of Commons today will include the first major vote on revising British abortion laws since then.
Opposition party chief David Cameron and several cabinet ministers advocate a lowering of the 24-week limit for abortions in Britain.
Legislators will vote today on whether to retain the current limit or lower it to 22, 20, or 16 weeks.
Lawmakers will also vote on whether to fully authorize the screening of embryos for genetic characteristics to create so-called savior siblings.
These are cases where parents seek to have a child with specific nondiseased characteristics so that he or she can donate tissues or organs to a diseased older sibling.
The proposed laws are in line with the latest scientific developments and would provide Britain's Human Fertilization and Embryology Authority with clearer guidelines.
The decisions are currently vulnerable to challenges in court, Lovell-Badge said.
In addition, lawmakers will decide on proposals to end the requirement for in-vitro fertilization clinics to consider the need for a child to have a father when evaluating patients.
Advocates say the change is necessary to enable lesbian couples and single women to gain easier access to fertility treatment.
But opponents insist the change fails to acknowledge the role of a father in a child's life.
Brown has said he will allow his Labor Party lawmakers to vote as they wish on the three controversial sections.
Three Catholic cabinet members—including Defense Secretary Des Browne—and about nine junior ministers are believed to have reservations about some aspects of the plans.
