The parents of a profoundly disabled baby girl, who a judge ruled last year should not be resuscitated if she fell critically ill, returned to the high court yesterday in an effort to overturn their daughter's "non ventilation" order.

Counsel for Debbie and Darren Wyatt told Mr Justice Hedley that 23-month-old Charlotte Wyatt, who is being cared for at St Mary's Hospital in Portsmouth, had "crossed an invisible line" and ventilation could now be appropriate.

"The overwhelming medical consensus is that there are situations in which it would be appropriate to ventilate her if she needed it," said David Wolfe, representing the couple. "Debbie and Darren are content for the court to make a declaration that in the very unlikely event of her suffering a cardiac arrest, it would be lawful for doctors not to ventilate her, but no wider than that." Urging the judge that his previous order "can no longer stand", Mr Wolfe said that Charlotte, who will be two next Friday, had made remarkable progress since doctors advised a year ago that it was no longer in her best interests to be revived.

Born three months prematurely on October 21 2003, Charlotte weighed 450g (1lb) and was about 13cm (5in) long. After a series of life-saving operations, she contracted an infection which left her blind, deaf and with extensive kidney damage.

Charlotte still spends large parts of the day under an oxygen canopy, but her parents say her lungs are now sufficiently strong that she can venture outside of the hospital in a specially adapted pushchair. Her kidneys have also been free of infection for six months.

In a newspaper interview published yesterday, Mrs Wyatt, who is seven months pregnant with her fourth child, said: "She is a completely different child to who she was last year. Everything they said she would never do she has done. Charlotte is only in pain now when she's teething ... She can see and hear and smile and experience pleasure."

Mrs Wyatt added that the hospital had told the couple they could take Charlotte home next March.

Under the order made last October and continued in April, Mr Justice Hedley authorised doctors not to ventilate Charlotte if she suffered an infection which had led or might lead to a collapsed lung, and which proved resistant to antibiotics. At the time, doctors advised that the ventilation process might kill Charlotte and deprive her of a peaceful death.

In August the Wyatts, who are committed Christians, failed to have that decision overturned by the court of appeal. This is their third attempt to force Portsmouth Hospitals NHS Trust to back down on their child's care.

In court yesterday Charlotte's consultant, who can only be identified as Dr K, said that the fact she had beaten the 5% chance of surviving a year she had been given last October, and with such an improvement, was remarkable. Although Charlotte still had chronic lung disease, cardiac arrest was unlikely and her chances of seeing out the winter were much improved.

But he said he could still foresee circumstances in which a court order would be necessary to allow medical staff to resist the Wyatts' demands for ventilation in the event of their ruling it was not in Charlotte's best interests. "Without a court order saying we were acting legally, I think it would be extremely serious and could cause very major problems for all the staff concerned," he said.

The Wyatts were entitled to wish and hope for a miracle and could not be criticised for that, but a decision whether or not to ventilate would be extremely difficult to make "at three in the morning," he added. "The worry is that Charlotte would be ventilated when the person knew in her heart it would not be right for her. That's a very difficult thing to do."

Earlier, the judge heard that the trust wanted to avoid a situation where the focus in the final hours of Charlotte's life was not on providing her with the best care but on making an application to the courts. The hearing continues.