The largest sperm bank in Britain is under investigation from health officials over claims they used sperm from the wrong donor after a gay couple had two children from different racial backgrounds.

The alleged mix-up at the IVF clinic only emerged after the birth of the couple's second child, who is of different race to the rest of the family.

The parents are said to be "devastated" at the alleged mistake as they had wanted their children to be genetically related by using the same sperm donor, The (London) Sunday Times reported.

"The damage to the [younger] child in the future, to both the siblings and the family unit could be quite catastrophic," said Caron Heyes, the couple's lawyer.

The Human Fertilisation and Embryology Authority, which regulates IVF clinics in the UK, began an investigation in January into the alleged blunder at the London Women's Clinic. A report will be considered by the watchdog's license committee on May 31.

More than 13,000 babies are born every year in the UK from IVF treatment, and while mix-ups are rare they are not unknown. In 2010, a couple lost a negligence lawsuit against a Belfast hospital after a white couple had a black child after receiving sperm from the wrong donor.

In 2002, a white couple had mixed-race twins after an Asian man's sperm was mistakenly used to fertilize the woman's eggs at a Leeds IVF clinic. "All we wanted was a family. Instead, we were landed with a nightmare that will last forever," the woman later said.