A sperm donor who unknowingly harboured 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.