The judges declared that courts could always intervene in medical matters in the child's interest - and when a child decided to refuse treatment, parents could also challenge their decision.
For 20 years, since the introduction of the Family Law Reform Act, 16 to 18-year-olds have generally been treated as adults when it came to medical care. They did not need their parents' consent.
Yesterday Lord Donaldson, Master of the Rolls, sitting with Lords Justice Balcombe and Nolan, argued that their ruling - made in the light of an immediate and serious threat to the girl's health - would, in practice, only apply to a small number of cases, mainly in a life-or-death situation.
But they accepted that, technically, their decision could, for example, allow the parents of a girl under 18 who had became pregnant to demand that she had an abortion whether or not she wanted one. Lord Donaldson said that while such a 'hair-raising' scenario was possible, medical ethics would in practice prevent it.
Lord Justice Nolan said: '. . .the present state of the law is that an individual who has reached the age of 18 is free to do with his life what he wishes, but it is the duty of the court to ensure so far as it can that children survive to attain that age.'
Lawyers and civil rights and children's rights groups said the that judgment would apply to all forms of treatment. What would happen, for example, if a 16-year- old refused suggested cancer treatment?
It was 'a disaster', Rachel Hodgkin of the Children's Legal Centre, said.
They were also critical that the court had refused J's lawyers leave to appeal to the House of Lords, when their ruling arguably undermined the rights conferred on young people by Parliament.