The parents of the profoundly disabled two-year-old Charlotte Wyatt won a high court ruling yesterday lifting an earlier declaration that doctors need not give her artificial ventilation if a life-threatening situation arises. The ruling from Mr Justice Hedley came as Darren and Debbie Wyatt were preparing to celebrate their daughter's second birthday.

Portsmouth Hospitals NHS trust, which cares for her at the city's St Mary's hospital, had sought the order in advance to avoid a last-minute court battle at a time of medical crisis. Her parents refused to accept that ventilation might not be in her best interests. But in the light of Charlotte's improvement, the NHS trust agreed yesterday that a year-old declaration that doctors would not be acting unlawfully if they decided it was not in Charlotte's best interests to ventilate her in an emergency should be lifted.

The ruling, however, still leaves it open to doctors to refuse artificial ventilation if their conscience dictates that it would be against the child's best interests.

Mrs Wyatt, 24, who is heavily pregnant with the couple's fourth child, and her husband, 33, say Charlotte's condition has improved to such an extent that she has now "crossed an invisible line", meaning that the use of artificial ventilation in certain circumstances would be justified.

As the couple left the high court in London to hold a birthday party for Charlotte, a statement issued on their behalf stated: "Darren and Debbie are very happy that the order hanging over Charlotte for over a year has been lifted."

Despite serious brain, lung and kidney damage, Charlotte has survived against the medical odds and is said to have made "remarkable progress". She weighed just one pound and was only five inches long when she was born three months prematurely at St Mary's hospital. But the judge said her prognosis was still "gloomy" and that as she grew her lung problems would make it harder to breathe.

The couple hope Charlotte, who has been hospitalised since birth, will be able to come home. But that cannot happen while she continues to need the headbox which provides extra oxygen to her lungs.

Mr Justice Hedley said the NHS trust had sought a new declaration that would give the doctors the last word should there be irreconcilable differences between them and the Wyatts.

Dr K - Charlotte's doctor, who cannot be named for legal reasons - had told the court: "I am convinced that the clinicians still need a declaration ... I am fearful that if treatment decisions were restored to the parents they would not be able to accept that we had reached the position where it was not in Charlotte's interest to be ventilated."

Later Dr K said: "I would not wish to ask any clinician to take a decision not to ventilate Charlotte without the support of a court order ... the history of the case suggests that any clinician who refused to ventilate Charlotte without a court order authorising such a step would face condemnation in the media, complaints to the police, a probable report to the General Medical Council and civil litigation."

But although the judge formally lifted the existing declaration, he said the law would not require a doctor to carry out treatment on a child which the doctor did not believe to be in the child's best interests. "I have tried to set out, in a way comprehensible to all, what I understand to be both the duties and also the limits on the duties of the treating clinician," Mr Justice Hedley said. "He does not take orders from the family any more than he gives them. He acts in what he sees as the best interests of the child: no more and no less. In doing so, however, parental wishes should be accommodated as far as professional judgment and conscience will permit, but no further."

David Lock, counsel for the NHS trust, said: "The judge has backed comprehensively the doctors' assessment of the position and has explained the doctors' rights to treat Charlotte in the way they consider correct, and not to have to follow the instructions of the parents in the event that what they have been asked to do is against the doctors' conscience."