A senior High Court judge has ruled that a black man is the legal father of mixed-race twins born to a white couple after the wrong man's sperm was accidentally used in IVF (in vitro fertilisation) treatment.

Last year, genetic tests established that the white woman who gave birth to the twins, known publicly only as 'Mrs A', is also their genetic mother. But, during IVF treatment, her eggs had been fertilised with the sperm of 'Mr B', a man from another couple undergoing fertility treatment on the same day.

In a previous hearing last November, Dame Elizabeth Butler-Sloss revealed that the black couple involved remain childless, although she stressed that there was no suggestion that the twins should be uprooted from their 'happy and loving environment' with the white couple.

In the latest ruling, Dame Butler-Sloss had been asked to decide which man - Mr B, the biological father, or Mr A, the social father - was the legal father of the children. The legal argument centred on whether Mr A gave his consent to the treatment. Under section 28 of the Human Fertilisation and Embryology Act 1990, social fathers can be recognised as the legal father of children born to their wives or partners following artificial insemination, unless they did not give consent. The court held in this case that Mr A had not consented to the use of Mr B's sperm, only his own.

Deciding that Mr B is the legal father means that Mr A will have to adopt the children if he wishes to become their legal parent. Mr and Mrs A are now considering this route having been refused leave to appeal the decision, although they can petition the Court of Appeal directly if they wish to. Mr and Mrs A have said they are disappointed with the ruling, while expressing sympathy for Mr and Mrs B. Lawyers representing Mr and Mrs B thanked Mr and Mrs A for their 'sympathy and understanding' and said his clients wished for time to reflect on the implications of the judgement.