Health experts have welcomed news that fertility doctors have been given permission to create the UK's first "three-parent" babies to prevent inherited disease.
Two women being treated at the Newcastle Fertility Centre at Life will now receive mitochondrial replacement therapy (MRT) after being deemed suitable by the Human Fertilisation and Embryology Authority (HFEA).
The aim of the treatment is to prevent women passing on defective genes in the mitochondria - tiny rod-like power plants in cells which supply energy.
But the technique, which involves giving a woman an IVF baby with DNA from three individuals, is highly controversial.
The baby will have nuclear DNA from its mother and father which define key characteristics such as personality and eye colour.
In addition, it will have a tiny amount of mitochondrial DNA provided by a female donor - the third "parent".
An estimated one in 200 children are born with defective mitochondrial DNA (mtDNA).
Britain became the first country in the world to formally allow MRT when the HFEA gave a cautious green light to the procedure last year.
Approval of the Newcastle clinic's facilities, equipment and staff was announced by the HFEA in March, but separate appraisals are still needed to assess the suitability of each candidate.
These have to take account of medical history and personal biology to ensure that the procedure is safe and likely to meet with success.
A HFEA spokeswoman said: "Our Statutory Approvals Committee has considered applications from the Newcastle Fertility Centre at Life, part of Newcastle upon Tyne Hospitals NHS Foundation Trust, for the use of mitochondrial donation in treatment for two patients, both of which have been approved."
Dr Jeremy Farrar, director of the Wellcome Trust, welcomed the news.
"It is wonderful that after more than a decade of research, public consultation, scientific and legal review, the first women are now taking the next steps to being able to have this treatment and a healthy baby," he said.
"Our best wishes go to families setting out on their exciting and momentous journey."
The director of the Progress Educational Trust, Sarah Norcross, said: "The granting of licences to treat two women in the UK is the latest milestone in seeking to help parents avoid passing on mitochondrial disease to their children.
"The pace at which these treatments are being rolled out may seem slow, but this highly regulated and measured approach will ensure the highest standards of treatment and follow-up research.
"Options which for many years have been tantalisingly out of reach to patients are now a step closer."
In 2015, MPs and peers paved the way for the change by voting to alter the Human Fertilisation and Embryology Act which sets the legal framework for fertility research and treatment.
Mitochondrial replacement has been made an exception to the general rule which outlaws tampering with "germline" inherited DNA.