A report from an influential think tank could help clear the way to IVF babies being born with DNA from three different people.
Outlawed techniques that give a baby DNA from a father, a mother and a woman donor to prevent inherited disorders are morally justified, says the Nuffield Council on Bioethics.
Their purpose is to stop the transmission of defective mitochondrial DNA (mDNA) from mothers to their babies.
Children born after the procedures would possess nuclear DNA inherited from their parents plus mitochondrial DNA (mDNA) from a woman donor.
Mitochondria are rod-shaped power plants in the bodies of cells that supply energy. They contain their own DNA, which is only passed down the maternal line.
Faulty mitochondrial genes can lead to a wide range of serious disorders including heart malfunction, kidney and liver disease, stroke, dementia, and blindness, as wells as premature death.
Around 6,000 adults in the UK are believed to be affected by mitochondrial diseases.
Controversy surrounds attempts to prevent such diseases through hi-tech variations of In-Vitro Fertilisation (IVF) treatment.
One technique, pronuclear transfer, involves transferring nuclear DNA out of a day-old embryonic cell containing defective mitochondria. The DNA is planted into another single-cell embryo whose mitochondria function normally.
The donor embryo's own nuclear DNA is discarded. However, it still contains the normal mitochondria of the woman whose egg was fertilised to create it.
As it grows, the embryo produces a baby with DNA from three sources - nuclear DNA from the original parents, plus a tiny amount of mitochondrial DNA from the woman egg donor.
Another technique, maternal spindle transfer (MST), is similar but involves transferring nuclear DNA from an unfertilised egg to a donor egg. The egg is then fertilised using the father's sperm.
Although such techniques are banned, they could be voted in by Parliament under existing legislation.
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A working group of the Nuffield Council on Bioethics decided that such treatments would be ethical, despite tampering with inherited DNA.
Although mDNA has important biological effects, it is not believed to have any impact on basic individual characteristics.
Dr Geoff Watts, who chaired the Council inquiry, said: "If further research shows these techniques to be sufficiently safe and effective, we think it would be ethical for families to use them if they wished to, provided they receive an appropriate level of information and support.
"They could offer significant health and social benefits to individuals and families, who could potentially live their lives free from what can be very severe and debilitating disorders.
"We would recommend that families commit to allowing long term follow-up of the resulting children, supported by a centrally-funded register of such procedures performed in the UK, which would be accessible to researchers over several decades."
He added: "We understand that some people concerned about the idea of germline therapies may fear that if such treatments for mitochondrial gene disorders were approved, a 'slippery slope' would be created towards comparable alterations to the nuclear genome.
"However, we are only talking about the use of these techniques in the clearly-defined situation of otherwise incurable mitochondrial disorders, under strict regulation."
Professor Frances Flinter, a clinical geneticist and member of the working group, pointed out that only 0.1% of affected children's DNA would be donated.
"As far as we know, mitochondrial genes alone create no unique identifiable genetic link between the child and donor," she said.
"The child's recognisable likeness to family members would come from their parents' nuclear DNA. Given these, and other reasons, we think it is both legally and biologically inaccurate to refer to a mitochondrial donor as a 'second mother' or 'third parent' to the child."
Anthony Ozimic, spokesman for the pro-life group Society for the Protection of Unborn Children (SPUC) said: "As with IVF and cloning, this mitochondrial technique may well lead to the developmental abnormalities. Creating embryonic children in the laboratory abuses them, by subjecting them to unnatural processes. These techniques are both destructive and dangerous and therefore unethical.
"The vast majority of embryonic children created in the laboratory are killed because they do not meet the 'quality control' requirements dictated by scientists involved in such increasingly macabre experiments.
"Scientists should abandon the spurious field of destructive embryo experimentation and instead promote the ethical alternative of adult stem cell research, which is already providing cures and treatments for an increasing number of conditions."
A statement from the Human Fertilisation and Embryology Authority, which regulates fertility treatment an research, said: "The HFEA welcomes this timely and important report from the Nuffield Council on Bioethics. We share the Council's belief that continuing public debate about these issues is important; the HFEA will lead this debate.
"We will launch a public consultation in September 2012, as part of a wider programme of public engagement throughout the year. Our aim is to facilitate a lively and informed debate in order to gauge public perceptions of the ethical and social issues surrounding the use of IVF techniques designed to prevent mitochondrial disease in treatment. We will report our findings to Ministers in Spring 2013."
Dr David King, director of the group Human Genetics Alert, accused scientists of acting like "Frankenstein".
He said: "The proposed techniques are both unnecessary and highly dangerous in the medium term, since they set a precedent for allowing the creation of genetically modified designer babies. But these considerations are ignored by the bioethics industry because of its flawed methodology and in its usual rush to embrace risky hi-tech 'solutions'."
Dr King added: "What worries most people about constructing a person in this way is the same thing that worries them about GM foods or human animal hybrids: the way that scientists treat nature as a set of infinitely exchangeable parts to be mixed and matched as necessary.
"Just as Frankenstein's creation was produced by sticking together bits from many different bodies, it seems that there is no grotesquerie, no violation of the norms of nature or human culture at which scientists and their bioethical helpers will balk. But such concerns cannot be admitted as real within the discourse of bioethics and can certainly never overcome its trump cards of 'medical progress' or simply 'increasing knowledge'."