Ms B, who has spent more than a year on a ventilator paralysed from the neck down, said she was delighted at the high court's ruling yesterday that she had the right to decide to have the life support machine switched off.

But, though the circumstances were dramatic, the case was never a cliffhanger. There was only one ruling Dame Elizabeth Butler-Sloss could have given.

The law is clear, not only in Britain, but confirmed by precedents in the US and Canada: a patient with the mental capacity to take decisions has the absolute right to refuse treatment. It makes no difference whether doctors think the decision is unreasonable or irrational or not one that they would advise. Respect for the patient's autonomy requires that it is the patient's values which prevail.

Ms B, a former senior social worker, is not saying that a life paralysed on a ventilator is not a life worth living; only that for her, with no immediate family to care for her, it is not worth living. As John Stuart Mill put it in On Liberty, "Over himself, over his own body and mind, the individual is sovereign."

Dame Elizabeth said: "Unless the gravity of the illness has affected the patient's capacity, a seriously disabled person has the same rights as a fit person to respect for personal autonomy. There is a serious danger, exemplified in this case, of a benevolent paternalism which does not embrace recognition of the personal autonomy of the severely disabled patient."

The judge said she was helped in reaching her judgment by an article from the Journal of Applied Philosophy by Dr Kim Atkins of the University of Tasmania. Dr Atkins said carers who criticised a patient for being insufficiently objective in refusing life-saving treatment were missing the point. The difficulty was not in becoming more objective, but in being appropriately subjective. "Insisting that a decision be made from a fully objective perspective can only produce a decision that is further from the patient's own point of view, not closer to it."

Dame Elizabeth said Ms B "valued the ventilator and her handicap as worse than death. Her decision was made against the advice offered and was not understood.

"Subjective values have to be taken into account. If at an earlier stage there had been an acknowledgment of a clash of values it might possibly have led to a different approach to management of the case."

Dame Elizabeth's judgment will not open the floodgates to physician-assisted suicide or to euthanasia, voluntary or involuntary. Contrary to the perceptions of Ms B's doctors, it will be her medical condition which kills her, not they, if they switch off the ventilator. When they put her on life support, it interrupted the natural course of a condition that would have killed her. When she comes off the ventilator, her illness will result naturally in her death.

Ms B is not even the first patient in Britain to go to court to have a ventilator switched off, though she is the first with a normal life expectancy. Legal research for her case turned up another in 2000, which had no publicity, in which a 19-year-old man with motor neurone disease who could communicate only by blinking his eyes was allowed to be detached from the ventilator keeping him alive. So clear was the law that the judge in that case said a court declaration was, strictly speaking, unnecessary, once it was decided that the patient, despite his limited ability to communicate, was competent to take the decision.

The mystery is that the NHS trust caring for Ms B continued to contest a case that it was doomed to lose and that will cost it over £100,000 in legal fees. Dame Elizabeth's ruling that the trust has been guilty of trespass to the person in treating Ms B unlawfully since last August was foreshadowed nearly a year ago in legal advice from the trust's own solicitor. The trust has been lucky to escape a substantial damages award. Ms B, as a former NHS worker, had no wish to deprive the service of funds, and was awarded nominal damages of £100.

Dame Elizabeth "had the greatest possible sympathy" for the position of the doctors caring for Ms B, who had grown to know and like her, and felt unable to switch off the ventilator personally. But Ms B had been put in an impossible position. The "one-way weaning process", which was suggested as a compromise, appeared to have been designed to help the carers, not Ms B. She would die over three weeks and not in a few hours, and "there would be a risk that she would die in discomfort and possibly in pain".

Dame Elizabeth laid down guidance for future cases, warning doctors that they must not confuse the issue of mental capacity with the nature of the decision made by the patient, however grave the consequences. "Those in charge must not allow a situation of deadlock or drift to occur," she said. "If there is no disagreement about competence but the doctors are for any reason unable to carry out the wishes of the patient, their duty is to find other doctors who will do so."