Modifications to inherited human DNA could be permitted in future in order to treat or prevent diseases that may be passed onto future generations, two major US scientific institutions have said.
The report on gene editing is a landmark since it amounts to an official sanctioning of medical research that aims to add, remove or replace DNA in human egg cells, sperm or embryos.
Many critics have argued that powerful new gene editing techniques should never be used to alter inherited DNA. They argue that such a move would be the start of a slippery slope leading to "designer" babies with selected features such as blue eyes, high intelligence or sporting prowess.
Gene editing, which effectively allows the precise "cutting and pasting" of DNA, is already used in basic research and clinical studies that involve non-heritable "somatic" cells.
Now the National Academy of Sciences and National Academy of Medicine have ruled that gene editing of the human "germline" - inherited DNA - should not be seen as a red line in medical research.
Future use of germline gene editing to treat or prevent disease and disability is a "realistic possibility that deserves serious consideration", the report says.
However, the two academies point out that the technology is not yet safe enough to justify testing it on the inherited DNA of human patients.
They add that gene editing for enhancement should not be allowed "at this time" - but do not rule it out completely.
A broad public debate should be held before permitting clinical trials, even those involving non-inherited DNA, for any purpose other than treating or preventing disease, the report says.
Professor Alta Charo, from the University of Wisconsin-Madison in the US, who co-chaired a study committee appointed by the academies to investigate the wider implications of gene editing, said: "Human genome editing holds tremendous promise for understanding, treating or preventing many devastating genetic diseases, and for improving treatment of many other illnesses.
"However, genome editing to enhance traits or abilities beyond ordinary health raises concerns about whether the benefits can outweigh the risks, and about fairness if available only to some people."
Currently research that involves modifying inherited genes in human embryos is not allowed in the US, and a number of other countries have signed an international convention that prohibits it.
Altering germline DNA is also banned in the UK, with one important exception. Parliament has ruled that inherited DNA in the mitochondria - tiny power plants in cells that supply energy - can be replaced if they are defective and the cause of devastating diseases that are passed down from mothers to their children.
Mitochondrial DNA makes up only about 0.1% of all the inherited genetic material in a human cell and does not affect key characteristics such as hair and eye colour or personality.
The academies said although heritable gene-editing trials must be approached with caution, "caution does not mean prohibition".
The study committee set out strict rules that would have to be met before advancing to clinical trials.
They include the absence of "reasonable" alternatives, restriction to editing genes that have been shown to cause or increase the risk of a serious disease or condition, the availability of credible data on risks and potential health benefits, "rigorous" oversight, and continued reassessment of "both health and societal benefits, with wide-ranging ongoing input from the public".
Dr David King, director of the watchdog body Human Genetics Alert, accused the two academies of taking "another step towards eugenic dystopia".
He added: "It is ironic that on Valentine's day, which is all about love, scientists want to make having babies part of the industrial machine. If scientists create GM babies, it will be impossible to avoid the 'designer babies' dystopia, because the line between therapy and enhancement has not been respected with any other medical technology, such as drugs, surgery etc.
"This step by scientists illustrates yet again that we cannot trust them to act responsibly. Human Genetics Alert calls for an international campaign against the creation of GM babies and human cloning."