British authorities ruled Wednesday that research using animal eggs to create human stem cells could go forward in principle.

The Human Fertilization and Embryology Authority decided to allow the research, which involves placing human DNA into cow or rabbit eggs that have had their genetic material removed. According to the ruling, projects would be decided on a case-by-case basis, said Paula Woodward, a spokeswoman for the regulator.

Experts have said such research, which is currently under way in the United States and China, is critical to unlocking treatments for Alzheimer's, Parkinson's and other genetic diseases. Scientists want to use animal eggs because the supply of human eggs is limited.

However, the research has raised ethical worries. The involvement of animals has caused concern among the public, while right-to-life advocates fear it could lead ultimately to genetically modified babies — despite the fact that the studies being considered would only allow development of eggs for a few days.

The research involves taking a cow or rabbit egg which no longer has its own DNA and injecting human genetic material. The egg is induced to divide, becoming a very early embryo from which stem cells could be extracted.

Some experts questioned whether residual animal traces might contaminate human DNA, thus invalidating the stem cell experiments.

Advocates of the research insist it would be a human embryo made in the shell of an animal egg, though a minute amount of animal genes remain. The resulting egg contains 13 animal genes compared with some 20,000-25,000 human genes.

Dr. David King, director of the independent watchdog group Human Genetics Alert, said allowing such research to go forward would be the first step toward producing genetically modified babies.