Seventeen British sperm donors have fathered more than 500 children between them, new figures show.

The startling numbers have led to fears that men could be unknowingly passing defective DNA to dozens of youngsters, because currently donors are not screened for faulty genes such as BRCA1/2 which increase the risk of ovarian and breast cancer.

It also raises the risk that siblings could unexpectedly meet and form relationships without realising they are related.

Although more than 18,000 children have been born who have at least nine other half-brothers or sisters, just 163 have registered on the Human Fertilisation and Embryology Authority (HFEA) Donor Sibling Link, which attempts to reunite them.

The new figures from the HFEA showed that 17 men had fathered at least 30 babies each between 1991 and 2015. A further 104 men have fathered between 20 and 29 babies, and 1557 between 10 and 19. More than 6,000 have created nine or fewer.