Britain’s test tube babies could have hundreds of half siblings they do not know about because of gaps in the country’s sperm bank system, a charity boss has claimed.

A rule stipulating that serial numbers on sperm donations from abroad are not shared in Britain is stopping Britons born via IVF from tracking down possible relatives, Donor Sibling Registry, a US-based charity, has revealed.

More than half of sperm used by families in the UK now comes from abroad in response to rising demand for fertility treatment, with the lion’s share imported from the US and the Netherlands.

Serial numbers on imported donations can include identifying information, but are typically withheld to protect the identity of the donor.

But Wendy Kramer, of the charity, said: “Lots of British families have no idea that they have half siblings or a donor willing to connect abroad, because they aren’t being given the originating donor number.