The sperm was not sold to UK clinics, but the BBC can confirm a "very small" number of British families, who have been informed, used the donor's sperm while having fertility treatment in Denmark.
Denmark's European Sperm Bank, which sold the sperm, said families affected had their "deepest sympathy" and admitted the sperm was used to make too many babies in some countries.Up to 20% of the donor's sperm contains the dangerous mutation that increases the risk of cancer .
The investigation has been conducted by 14 public service broadcasters, including the BBC, as part of the European Broadcasting Union's Investigative Journalism Network.=
The sperm came from an anonymous man who was paid to donate as a student, starting in 2005. His sperm was then used by women for around 17 years.
He is healthy and passed the donor screening checks. However, the DNA in some of his cells mutated before he was born.It damaged the TP53 gene – which has the crucial role of preventing the body's cells turning cancerous.
Most of the donor's body does not contain the dangerous form of TP53, but up to 20% of his sperm do.However, any children made from affected sperm will have the mutation in every cell of their body.
Infographic explaining how a TP53 gene mutation in the donor's sperm can lead to cancer risk in children conceived via IVF. Top-left panel shows a DNA strand with text: “TP53 helps the body prevent cancer, but mutations can stop it working.
” Top-right panel shows a human figure next to sperm illustrations with text: “The donor is unaffected by the mutation, but it is present in up to a fifth of his sperm.” Bottom-left panel shows a sperm cell inside a circle with text:
“If mutated sperm are used in IVF, the mutation would end up in every cell in the child’s body.” Bottom-right panel shows a silhouette of a child with text above saying:
“The resulting child has an up to 90% risk of developing cancer in their lifetime, including - breast cancer, bone cancers, brain tumours, childhood cancers like leukaemia”.
This is known as Li Fraumeni syndrome and comes with an up to 90% chance of developing cancer, particularly during childhood as well as breast cancer later in life.
"It is a dreadful diagnosis," Prof Clare Turnbull, a cancer geneticist at the Institute of Cancer Research in London, told the BBC. "It's a very challenging diagnosis to land on a family, there is a lifelong burden of living with that risk, it's clearly devastating."
MRI scans of the body and the brain are needed every year, as well as abdominal ultrasounds, to try to spot tumours. Women often choose to have their breasts removed to lower their risk of cancer.
The European Sperm Bank said the "donor himself and his family members are not ill" and such a mutation is "not detected preventatively by genetic screening". They said they "immediately blocked" the donor once the problem with his sperm was discovered.
Doctors who were seeing children with cancer linked to sperm donation raised concerns at the European Society of Human Genetics this year.
They reported they had found 23 with the variant out of 67 children known at the time. Ten had already been diagnosed with cancer.
Through Freedom of Information requests and interviews with doctors and patients we can reveal substantially more children were born to the donor.
The figure is at least 197 children, but that may not be the final number as data has not been obtained from all countries.
It is also unknown how many of these children inherited the dangerous variant.