LONDON » Britain’s fertility regulator has approved a scientist’s request to edit the human genetic code in an effort to better understand how embryos develop — but critics fear the new technique crosses too many ethical boundaries.

The Human Fertilisation and Embryology Authority announced today it has granted a research application to a team led by scientist Kathy Niakan to try to understand the genes that human embryos need to grow successfully, which could potentially help prevent miscarriages.

Scientists say gene-editing techniques could one day lead to treatments for conditions like HIV, which causes AIDS, or inherited diseases like muscular dystrophy and sickle cell disease.

Niakan, of the Francis Crick Institute, plans to use gene editing to analyze the first week of an embryo’s growth.

This research will “enhance our understanding of (in vitro fertilization) success rates by looking at the very earliest stage of human development,” said Paul Nurse, director of the Francis Crick Institute.

Peter Braude, an emeritus professor of obstetrics and gynecology at King’s College London, said the mechanisms being investigated by Niakan and her colleagues “are crucial in ensuring healthy, normal development and implantation” and could help doctors refine fertility treatments.

Braude is not connected to Niakan’s research.

Gene editing involves using tools to precisely edit genes inside living cells. Scientists typically find specific sections of DNA to slice and repair or replace, much like a biological version of cut-and-paste software. There are a few methods but the technique known as CRISPR-Cas9 is a relatively fast, cheap and simple method that many researchers are keen to try it.

Yet critics warn that tweaking the genetic code this way could eventually produce a slippery slope that eventually leads to so-called “designer babies,” where parents not only aim to avoid inherited diseases, but seek taller, stronger or smarter children with specific physical characteristics.

“This is the first step on a path that scientists have carefully mapped out towards the legalization of (genetically modified) babies,” David King of advocacy group Human Genetics Alert said last month when the British fertility regulator met on the topic.

Around the world, laws and guidelines vary widely about what kind of research is allowed on embryos, since that could change the genes of future generations. In the U.S., the National Institutes of Health won’t fund this kind of research but private funding is allowed.

Last year, Chinese researchers made the first attempt at modifying genes in human embryos. Their laboratory experiment didn’t work — the embryos weren’t viable from the very start of the experiment— but raised the prospect of altering genes to repair the genes of future generations.

The CRISPR-Cas9 gene-editing technique was developed partly in the U.S. and scientists there have experimented with the method in animals and in human cells in the laboratory. The method has not been used for any kinds of patient therapies just yet, although first-step testing of an initial technique has begun by Sangamo Biosciences in Richmond, California, which is developing an HIV treatment based on gene editing.

At an international meeting in Washington last year, scientists agreed that attempts to alter early embryos as part of laboratory research should be allowed but that the technique was nowhere near ready for use in pregnant women.

Last year, British lawmakers voted to allow scientists to create babies from the DNA of three people to prevent children from inheriting potentially fatal diseases from their mothers. In doing so, it became the first country in the world to allow genetically modified embryos to be transferred into women.