Sperm donated in the UK is being exported and can be used to create large numbers of children across multiple countries, contradicting a strict 10-family limit that applies in the UK, experts warn.

A legal loophole means that, while a single donor can be used to create no more than 10 families in UK fertility clinics, there are no restrictions on companies making sperm or eggs available for additional fertility treatments abroad.

With the lifting of donor anonymity and the ability to track down genetic relatives on DNA testing sites, this raises the prospect of some donor-conceived children navigating relationships with dozens of biological half-siblings across Europe.

Prof Jackson Kirkman-Brown, chair of the Association for Reproductive and Clinical Scientists (ARCS), is among those calling on the Human Fertilisation and Embryology Authority (HFEA) to tighten restrictions.

“If you believe that it’s necessary to enforce the 10-family limit in the modern world then logically that should apply wherever the sperm are from,” said Kirkman-Brown, who is also director of the Centre for Human Reproductive Science at the University of Birmingham. “There is data showing that some of the children who find the really big families struggle with that.”