Fifteen sperm donor superdads have fathered 20 or more kids each.
Fertility watchdogs revealed that they are among 500 secret donors responsible for fathering more than 6,100 children.
But the youngsters are unlikely to find out who their biological father is - or meet their half-brothers and half-sisters.
The dads donated their sperm before a 2005 law change which ended anonymity - so their identity will be kept secret unless they opt to reveal it.
The UK Human Fertilisation and Embryology Authority (HEFA) figures reveal that exactly 500 anonymous donors have fathered 10 or more kids each - a total of 6,111. Of these, eight have fathered 20 children and seven even more.
The rules allow sperm from a donor to be given to up to 10 different women. But donors are not the legal father - unless it was done through an unlicensed clinic - and have no rights and obligations.
HEFA said the superdads had offered the only hope to infertile couples.
A spokesperson added: "Donation is an altruistic act. Sperm from donors can only be used to create to up to ten families and the recruitment process is and will remain a rigorous one."
More on Parentdish: Sperm donor who fathered 14 kids is 36-year-old virgin
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