A pioneering technique to 'fix' an embryo's faulty DNA was unveiled by scientists last night.
In a world first, they used gene-editing to cut out DNA from a fertilised egg. The embryo then repaired itself, replacing the mutated material with healthy cells.
The technique worked on three quarters of the 58 embryos it was tried on. It has the potential to revolutionise medicine and could lead to the eradication of inherited diseases such as cystic fibrosis and breast cancer.
Campaigners warned however that it might also open the door to "superior designer babies", with genes modified to improve physical appearance, strength or even intelligence.
"What concerns me most is that we will start making babies to order, and then expecting them to perform according to the way we have genetically designed them," said Dr David King, of the campaign group Human Genetics Alert.
Other experts said the research was remarkable and had taken gene-editing from "future fantasy to the world of possibility".
Professor Simon Fishel, founder of Care Fertility, said: "This is exciting research that in time may herald a new approach for correcting embryos carrying devastating genetic disorders.
"Such technology would ensure that those families afflicted with such diseases no longer need worry about passing it down the family line."
The breakthrough was led by scientists from Oregon Health and Science University in Portland using the gene-editing tool Crispr-Cas9, which works like "molecular scissors".
It occurs when the walls of the heart become thickened and stiff, making it more difficult for the organ to pump blood around the body.
None of the embryos were permitted to develop beyond five days after conception. Had they lived, the babies would no longer develop the heart condition or pass it on to their own children.
One of the leading figures in the research, Dr Shoukhrat Mitalipov, from OHSU, said: "Every generation on would carry this repair because we've removed the disease-causing gene variant from that family's lineage.
"By using this technique, it's possible to reduce the burden of this heritable disease on the family and eventually the human population."
Dr Mitalipov also hinted that first steps towards bringing the treatment to patients could take place in the UK under the direction of the fertility regulator the Human Fertilisation and Embryology Authority.
He said: "Maybe the HFEA might take a lead on this, but I'm quite sure before these clinical trials can go on they have to go through, I believe, Parliament to change a law. So there is still a long road ahead, particularly if you want to do it in a regulatory way."
Professor Peter Braude, from King's College London, said the work showed how rapidly the field of gene editing had progressed.
He added: "Although use of this method clinically would not be allowed under current legislation in this country, with this paper the possibility of germline genome editing has moved from future fantasy to the world of possibility.
"The debate about its use, outside of fears about the safety of the technology, needs to run to catch up."
But Josephine Quintavalle, of the campaign group Comment on Reproductive Ethics, said the research was of an 'extremely questionable kind' and warned of the risk that it could one day be used to create 'superior' babies.
In a statement, the HFEA said: "UK researchers can apply for a licence to edit human embryos in research, but offering it as a treatment is currently illegal, the Daily Mail reports.
"Introducing new, controversial techniques is not just about developing the science - gene editing would need to offer new options to couples at risk of having a child with a genetic disease, beyond current treatments like embryo testing.
"High quality public discussion about the ethics of new treatments, expert scientific advice and a robust regulatory system are crucial when considering new treatments of this kind."
The illnesses it could tackle
Parents at risk of passing on genetic conditions to their children can only currently prevent it with embryo screening.
This means undergoing IVF, during which doctors cherry-pick embryos that do not appear to carry the mutated gene. But it is a genetic lottery and, devastatingly, all embryos could be carriers.
The new research offers hope of cutting out more than 10,000 conditions involving genetic faults inherited from one parent.
These single-gene conditions include Huntington's disease - an incurable condition which causes progressively worsening brain damage with tremors, personality change and mobility problems.
There is also Marfan syndrome, a disorder of the body's connective tissues that can cause vision and heart problems.
Achondroplasia, a form of dwarfism, and type 1 neurofibromatosis, which affects one in 3,000 babies and causes tumours to grow along their nerves, are also caused by a single gene.
Diseases involving genes from both parents, such as cystic fibrosis, would be far more challenging at this stage.