A sperm donor who unknowingly harbored a genetic mutation that dramatically raises the risk of cancer has fathered at least 197 children across Europe, a major investigation has revealed.
Some children have already died and only a minority who inherit the mutation will escape cancer in their lifetimes.
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.
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.
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.
Dr Edwige Kasper, a cancer geneticist at Rouen University Hospital, in France, who presented the initial data, told the investigation: We have many children that have already developed a cancer.
We have some children that have developed already two different cancers and some of them have already died at a very early age.
Céline, not her real name, is a single-mother in France whose child was conceived with the donor's sperm 14 years ago and has the mutation.
She got a call from the fertility clinic she used in Belgium urging her to get her daughter screened.
She says she has absolutely no hard feelings towards the donor but says it was unacceptable she was given sperm that wasn't clean, that wasn't safe, that carried a risk.
And she knows cancer will be looming over them for the rest of their lives.
We don't know when, we don't know which one, and we don't know how many, she says.
I understand that there's a high chance it is going to happen and when it does, we'll fight and if there are several, we'll fight several times.
The donor's sperm was used by 67 fertility clinics in 14 countries. The sperm was not sold to UK clinics.
However, as a result of this investigation, the authorities in Denmark notified the UK's Human Fertilisation and Embryology Authority (HFEA) on Tuesday that British women had travelled to the country to receive fertility treatment using the donor's sperm. Those women have been informed.
There is no law on how many times a donor's sperm can be used worldwide. However, individual countries do set their own limits.
The European Sperm Bank accepted these limits had unfortunately been breached in some countries and it was in dialogue with the authorities in Denmark and Belgium.
In Belgium, a single sperm donor is only supposed to be used by six families. Instead, 38 different women produced 53 children to the donor.
The UK limit is 10 families per donor.
Prof Allan Pacey, who used to run the Sheffield Sperm Bank, stated countries had become dependent on big international sperm banks and half the UK's sperm was now imported.
He emphasized that the case raises crucial questions about genetic screening and regulation in sperm donation practices, underlining the challenges of ensuring donor safety.
