Mr and Mrs A must learn to live with Mr B

It is the kind of dilemma that might have fascinated Shakespeare: who is the father of twins born to a couple, Mr and Mrs A, after the wife's eggs were mistakenly fertilised by sperm from another man, Mr B? The question has far-reaching legal, ethical and human consequences, which is why it went before the country's leading family judge last week. Dame Elizabeth Butler-Sloss must have felt like Solomon, who was also asked to decide a tricky question about parentage, when she heard the case at the High Court's family division.

Ironically, the dispute arose as the result of treatment that has been accused of allowing couples to produce "designer" babies. Mr and Mrs A, who are white, have mixed-race twins; Mr and Mrs B, who are black, remain childless, although Mr B is undoubtedly the biological father of the twins. Both couples might have remained in ignorance of the mistake if their different races had not made it visible; the Human Fertilisation and Embryology Authority has admitted that mistakes in fertility clinics have probably resulted in couples bringing up children fathered by strangers.

It has already happened in the Netherlands, where a woman gave birth to non-identical twins in 1993 after her husband's sperm was accidentally mixed with that of a man from the Dutch Antilles. The first thing to be said about such cases, apart from the fact that they create traumatic dilemmas for the couples involved, is that they bring into the public domain something that has been happening for centuries. If DNA testing was routinely carried out on the general population, there is little doubt that a substantial minority would discover that the man they regarded as their father was not biologically related to them.

For centuries, wives have conceived as a result of extra-marital affairs and even, in a precursor of modern fertility treatment, quietly had sex with someone other than their husband after attempts at conception in the marital bed have failed. This is one instance where the knowledge brought by scientific advances may be regarded as a mixed blessing, and it is hard to know whether Mr B – who was unaware that he had become a father until contacted by the clinic months later – will be happier for the revelation.

That does not mean that the clinic was wrong to tell him, for the most important factor in such cases, as the judge recognised on Wednesday, is the welfare of the children. Mr and Mrs A are to apply to adopt the twins after the judge ruled that Mr B is their biological father, ensuring that the children retain essential information about their "paternal identity". The ruling reflects a welcome shift away from the old assumption that adults' rights are paramount to a realisation that children, whether adopted or the product of assisted conception, need to know the identity of their biological parents.

What is also going on here, even though it is painful for the people who find themselves unwitting guinea-pigs in a reordering of law and ethics, is a reappraisal of the significance of fatherhood. When Joe Orton quipped in Entertaining Mr Sloane that "It's all any reasonable child can expect if the dad is present at the conception", he was reflecting a widespread assumption that fathers tend to be less engaged than mothers with their children. In the 1970s, feminists unwittingly encouraged this assumption with intense debates about the mother-child relationship, leaving the paternal bond out in the cold.

I don't think such neglect is tenable any longer, and last week's judgment dealt it a death blow. But it went even further than that, making a pioneering distinction between different types of parent – the "social and psychological father", in this case Mr A, and the legal and biological father, Mr B. In effect, the court was arguing that there are instances where children may have more than two parents, all of whom make a contribution to their identity.

This may be an uncomfortable conclusion for family-values campaigners, who like such things to be clear-cut, but it is both humane and sensible. The family is being radically reshaped and many children already grow up in unconventional households, with a nexus of caring adults. Last week's decision demonstrates that the family courts are ahead of policy-makers in this respect, recognising that the welfare of children is much more important than a narrow definition of mothers and fathers.