The first ever genetically modified (GM) human embryo has been created by researchers at Cornell University in New York.
The GM embryo was produced to study how early cells and diseases develop, but the scientists destroyed it just after five days.
However, the breakthrough has brought with it major concerns. The British regulator, the Human Fertilisation and Embryology Authority (HFEA), has even cautioned that such controversial experiments may lead to "large ethical and public interest issues".
The news has come days before MPs are scheduled to debate legislation enabling scientists to use similar techniques in the country.
Usually the genes added to embryos or reproductive cells, such as sperm, will affect all cells in the body and will be passed on to future generations.
This method has implications to be used for correcting genes, which cause diseases such as cystic fibrosis, haemophilia and even cancer. This means that any gene that has been identified could be added to embryos.
However, ethicists have cautioned against genetically modifying embryos as it may lead to the addition of genes for desirable traits such as height, intelligence and hair colour.
But, The Human Fertilisation and Embryology Bill, scheduled to have its second reading this week, will possibly make it legal to create GM embryos in Britain. However, GM embryos are allowed to be created only for research and the bill will ban implantation in the womb. But the ethicists have claimed that the legislation could be relaxed in the future.
The HFEA has claimed that it is trying to prepare scientists to apply for licences to create GM embryos.
"The bill has taken away all inhibitions on genetically altering human embryos for research. The Science and Clinical Advances Group [of the HFEA] thought there were large ethical and public interest issues and that these should be referred for debate," Times Online quoted the paper, published by the authority, as stating.
Led by Nikica Zaninovic, the Cornell team, used a virus to add a gene, a green fluorescent protein, to an embryo left over from in vitro fertilisation.
However, Dr David King, director of Human Genetics Alert, warned: "This is the first step on the road that will lead to the nightmare of designer babies and a new eugenics. The HFEA is right to say that the creation and legalisation of GM embryos raises 'large ethical and public interest issues' but neglects to mention that these have not been debated at all."
He added: "I have been speaking to MPs all week and no one knows that the government is legalising GM embryos. The public has had enough of scientists sneaking these things through and then presenting us with a fait accompli."
The research was presented at a meeting of the American Society of Reproductive Medicine last year but details have emerged only after the HFEA highlighted the work in a review of the technology.