When ten-year-old Michael Williams climbs into his father Keith's arms for a goodnight cuddle, he will often ask the same dreaded question: 'Why am I brown?' Looking up into his father's blue eyes and taking in his light brown hair and fair skin, his son will then ask: 'How can I make myself lighter, like you?'

Keith doesn't know how to answer these questions, so he'll make a joke about how Michael was delivered by a stork or that he was found under a gooseberry bush. Anything to avoid having to tell him the truth.

For the reality is that not only was Michael conceived via IVF using donor sperm, but there was a terrible mix-up when the wrong sperm was mistakenly used by the hospital fertility clinic the couple attended in their hope of becoming parents.

The result is that while Keith, 47, and his 46-year-old wife Catherine are white, their son is black. To add to their distressing situation, their 13-year-old daughter Susan, conceived three-and-a-half years earlier than their son in the same manner - and bizarrely with the same sperm donor - is light-skinned.

Instead of fertilising Catherine's eggs with 'White Caucasian' donor sperm, as the couple had requested and, they say, been promised, a staff member at the Regional Fertility Centre at Belfast's Royal Victoria Hospital accidentally used donor sperm marked 'Caucasian Cape Coloured'.

South African in origin, the sperm's racial make-up is a mixture of white, black and Malay, which means children born from it can be either black or white. This accounts for the difference in colour between the Williams siblings, who - although born three years apart - came from the same batch of fertilised eggs.

In a letter dated October 2003 to the Williams family - to protect the children's identities, the Mail has changed their names - the Regional Fertility Centre offered the couple an 'unreserved apology' for the mistake over the labelling.

It stated: 'Normal practice would be that only sperm from White Caucasian donors would be requested, unless a couple of a different race were being treated.'

Even more disturbingly, the hospital admitted the Williamses were among a number of couples treated at the Regional Fertility Centre (RFC) who, according to the letter, also 'achieved pregnancy as a result of using a particular batch of such donor sperm' - presumably with the same unexpected consequences.

Signed by Dr David Boyle - referred to as the 'Person Responsible' at the RFC - the letter concluded: 'We have taken this situation very seriously and have put systems and checks in place to ensure that it should not occur again.'

Now, in what is believed to be the first legal case of its kind in Britain, the Williamses are suing the Belfast Health and Social Care Trust (formerly the Royal Group of Hospitals Trust) for damages for their mental distress, social discredit and breach of contract under the Supply of Goods and Services Act 1982.