Story highlights Scientists can now alter the DNA of human embryos in their earliest stages

It is hoped that the research will help our understanding of how fertilized eggs develop

There are concerns that the program could lead the way to "designer babies"

(CNN) British scientists have been given the green light to genetically modify human embryos, for the first time in the nation's history.

The landmark decision means scientists will now be allowed to alter the DNA of embryos, for research purposes only.

It remains illegal for these genetically altered embryos to be implanted in a woman. It is hoped the experiments will improve our understanding of the earliest stages of embryo development.

The research, which was approved by the Human Fertilization and Embryology Authority, will use excess embryos donated by couples who have had in vitro fertilization treatment.

Scientists will be focusing on the first seven days of a fertilized egg's growth. In these early days, a fertilized egg evolves from a single cell to around 250 cells.