The treatment aims to avoid women passing on defective genes in the mitochondria and prevent children inheriting conditions which can cause serious illness and death.
Experts describe mitochondria as the powerhouses of cells, which help convert energy from food into something the cell can use.
When mitochondria fails, symptoms in children can include poor growth, poor muscle tone, weakness, failure to thrive, spasms and a slowdown in progress or a slow deterioration.
Some families have lost several children to inherited mitochondrial diseases and the new technique, mitochondrial donation treatment which is a modified form of IVF, is seen as their only chance of having a healthy child.
The technique which resulted in the birth of at least one child involves giving a woman an IVF baby with DNA from three people.
Most of the child's DNA comes from their two parents, but about 0.1 per cent has come from a third person — another woman.
The baby has nuclear DNA from its mother and father, which define key characteristics such as personality and eye colour. The DNA from the second woman only affects the mitochondria, and does not affect other key traits in the child.
“More than one but fewer than five babies” have been born in the UK using the treatment, the Human Fertilisation and Embryology Authority said, according to a freedom of information request submitted by The Guardian.
It said giving an exact number “could lead to the identification of a person to whom the HFEA owes a duty of confidentiality”. The data was accurate up until April 20.
The newspaper reported that the authority did not give further details to avoid families being identified.
The National Health Service trust behind the procedure said it would not be able to give any more information due to the fact that the treatment is the subject of a study currently going through peer review.
A Newcastle Hospitals NHS Foundation Trust and Newcastle University official told The National: “Academic publications have been submitted and are going through the process of scientific peer review. This means we are unable to comment further at this time in order to avoid prejudicing the process.”
“Mitochondrial donation treatment offers families with severe inherited mitochondrial illness the possibility of a healthy child,” said Peter Thompson, chief executive of the HFEA.
“The UK was the first country in the world to allow mitochondrial donation treatment within a regulatory environment.
“The HFEA oversee a robust framework which ensures that mitochondrial donation is provided in a safe and ethical manner.
“All applications for treatment are assessed on an individual basis against the tests set out in the law and only after independent advice from experts.
“These are still early days for mitochondrial donation treatment and the HFEA continues to review clinical and scientific developments.”
In the UK, MPs and peers paved the way for the change by voting in 2015 to alter the Human Fertilisation and Embryology Act, which sets the legal framework for fertility research and treatment.
But the world's first three-parent baby was born in 2016 in the US after scientists removed the nucleus from a healthy donor egg and replaced it with one taken from a woman who carried a gene for rare neurological disease called Leigh syndrome.
According to the journal Nature, the scientists then “fertilised the modified egg with the father’s sperm before implanting it into the mother’s uterus”.
However, the boy still carries traces of his mother's mitochondrial DNA — 2 per cent in his urine and up to 9 per cent on one area of skin. It is not known how this will affect his health as he ages.
News of the UK births was welcomed by Progress Education Trust, a charity which helps people affected by infertility and genetic conditions. It has campaigned for mitochondrial donation to become available in the UK.
Speaking to BBC Radio 4 on Wednesday, its director, Sarah Norcross, said: “Mitochondrial diseases impact multiple organs, or they can impact multiple organs in the body.
“There is not really a hope of a cure for these children who are severely affected and it is heartbreaking for these couples who have to watch their baby die.”
She said the secrecy surrounding the births, which came to light only after the FOI request, probably stems from the publicity generated by the first IVF baby in 1978, Louise Brown.
“There were bomb scares at the hospital that were phoned in by journalists to try and get in there and find out and break the story. But I think Newcastle were really nervous about that behaviour being repeated,” Ms Norcross said.
“I would very much hope that the team at Newcastle have a paper in progress already.
“So let’s hope if it is in progress it will be published soon, so there can be greater scrutiny of what’s happened here, and greater understanding.
“I dearly hope more than anything else that this is a good news story for the affected families and they have the opportunity to have a healthy family, which is all that anybody wants, really.”
What is mitochondrial disease?
During conception, half of the child’s DNA comes from their father and half from their mother. But a child always receives their mitochondrial DNA from their mother, regardless of their gender.
It is used to trace genetic heritage, and suggests the first humans originated in Africa about 200,000 years ago from an ancestor referred to as “mitochondrial Eve”.
Mitochondria are found in 90 per cent of our cells, so there are literally hundreds of different mitochondrial diseases because mitochondria perform so many functions in the body. Symptoms can include loss of muscle co-ordination and weakness, issues with vision or hearing and learning disabilities.
Mitochondrial diseases can be present at birth and occur at any age.
Common conditions believed to involve some level of mitochondrial dysfunction include Parkinson’s disease, Alzheimer’s disease, diabetes, bipolar disorder, schizophrenia and autism.
The history of IVF
IVF was first used in the 1950s on animals, with baby rabbits the first to be born using the process that decade.
Early experiments in the 1970s in humans failed but 1978 saw the first successful IVF pregnancy and live birth, of Louise Brown, who is still alive and well.
The procedure became mainstream in the 1980s and by 2012, more than 5 million babies had been born using the technique.
Currently around 2 per cent of all children born in the UK are conceived using IVF technology, according to the Bristol Centre for Reproductive Medicine.
IVF techniques have become more advanced over the years. And in 2017, the HFEA gave a cautious green light to use mitochondrial donation.
Doctors at the Newcastle Fertility Centre at Life were given permission to give two women mitochondrial replacement therapy the following year.
Two techniques for mitochondrial donation have been approved by parliament.
These are maternal spindle transfer (MST) and pronuclear transfer (PNT).
In both techniques, eggs or embryos are created using the nuclear genetic material from parents, plus a very tiny amount of healthy donated mitochondria.
In MST, the nuclear genetic material is removed from the eggs and transferred into donated eggs which have had their nuclear genetic material removed, according to details from the HFEA.
The eggs are then fertilised with sperm to create embryos.
In PNT, eggs are fertilised with sperm in a lab to create embryos. The nuclear genetic material within each embryo is then transferred into embryos created using donated eggs and sperm. The nuclear genetic material will have been removed from the donated eggs.
Anyone at very high risk of passing serious mitochondrial disease on to their children could be eligible for treatment.