Sperm from donor with cancer-causing gene was used to conceive almost 200 children

 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 and is now the deputy vice president of the Faculty of Biology Medicine and Health at the University of Manchester, said countries had become dependent on big international sperm banks and half the UK's sperm was now imported.

He told the BBC: "We have to import from big international sperm banks who are also selling it to other countries, because that's how they make their money, and that is where the problem begins, because there's no international law about how often you can use the sperm."

He said the case was "awful" for everybody involved, but it would be impossible to make sperm completely safe.

"You can't screen for everything, we only accept 1% or 2% of all men that apply to be a sperm donor in the current screening arrangement so if we make it even tighter, we wouldn't have any sperm donors – that's where the balance lies."

This case, alongside that of a man who was ordered to stop after fathering 550 children through sperm donation, has again raised questions over whether there should be tougher limits.

The European Society of Human Reproduction and Embryology has recently suggested a limit of 50 families per donor.

However, it said this would not reduce the risk of inheriting rare genetic diseases.

Rather, it would be better for the wellbeing of children who discover they are one of hundreds of half-siblings.

"More needs to be done to reduce the number of families that are born globally from the same donors," said Sarah Norcross, the director of the Progress Educational Trust, an independent charity for people affected by infertility and genetic conditions.

"We don't fully understand what the social and psychological implications will be of having these hundreds of half siblings. It can potentially be traumatic," she told BBC News.

The European Sperm Bank said: "It is important, especially in light of this case, to remember that thousands of women and couples do not have the opportunity to have a child without the help of donor sperm.

"It is generally safer to have a child with the help of donor sperm if the sperm donors are screened according to medical guidelines."

Sarah Norcross said these cases were "vanishingly rare" when you consider the number of children born to a sperm donor.

All of the experts we spoke to said using a licensed clinic meant the sperm would be screened for more diseases than most fathers-to-be are.

Prof Pacey said he would ask "is this a UK donor or is this a donor from somewhere else?"

"If it's a donor from somewhere else I think it's legitimate to ask questions about has that donor been used before? Or how many times will this donor be used?"