Legal permission will no longer be needed to withdraw treatment from patients in permanent vegetative state, the Supreme Court has ruled.
It will now be easier to withdraw food and liquid to allow such patients to die across the UK.
When families and doctors are in agreement, medical staff will be able to remove feeding tubes without applying to the Court of Protection.
Lady Black ruled there was no violation under the Human Rights Convention.
But anti-assisted dying campaigners said vegetative patients are "effectively going to be starved and dehydrated to death".
Previously the Court of Protection has ruled on cases but the process can take months or years, and it costs health authorities about £50,000 in legal fees to lodge an appeal.
The ruling could have an impact on the thousands of families whose loved ones are in a vegetative state.
There is estimated to be 1,500 new cases each year in England and Wales which would be affected, with about 3,000 of these patients alive at any one time.
The case was brought to the Supreme Court after a banker in his 50s suffered a heart attack, resulting in severe brain damage.
The man, known as Mr Y, was unresponsive after his heart attack last year and there was no chance of recovery.
He has since died but the case continued so that a court ruling could be made.
His family and his doctors agreed it would be in his best interests to allow him to die by withdrawing his feeding tube.
The NHS trust asked the High Court to declare that it was not necessary to apply to the Court of Protection for a decision when the doctors and the family all believe it is in the patient's best interests.
The judge agreed, but the official solicitor appealed on behalf of Mr Y – an appeal which has now been dismissed
Analysis: 'Judgement will divide opinion'
By BBC legal correspondent Clive Coleman
For years – so long as relatives agree, and it's in the best interests of a patient in a minimally conscious or vegetative state – doctors have been able to withdraw all sorts of treatment that will result in the end of someone's life.
These include, for instance, the withdrawal of life-saving dialysis. Doctors do not need the permission of a court to be able to do this.
However, withdrawing food and water – the most basic requirements for life – has been handled differently. Since the case of Hillsborough survivor Anthony Bland in 1993, it has been regarded as a matter of practice that doctors must seek the approval of a court, even when they and relatives agree withdrawal would be in the best interests of the patient.
It has been treated as an exception, in part, perhaps, because of the emotional and psychological significance of the decision to remove sustenance from a person.
This has resulted, some experts believe, in individuals spending longer on life support in a vegetative state than was necessary because hospitals have shied away from going to court due to the expense and bureaucracy involved.
Monday's ruling makes clear that courts need not be involved in these sorts of cases, so long as doctors and families are in agreement, and it is in the best interests of the patient.
However, the judgement cuts across ethical and religious beliefs and will divide opinion.
Some will see it as compassionate and humane, others the removing of a vital legal safeguard for a highly vulnerable group.
The mother of a woman whose care was ended after a Court of Protection ruling welcomed the Supreme Court's judgement and said it would make "a vast difference".
Jean Simpson's daughter Jodie, 38, took an overdose at home in Barrow in Furness in 2012.
It took several years for Jodie to be assessed as being in a permanent vegetative state, and after two assessments she was eventually moved to a hospice in 2016, where she died.
Jean said if the Supreme Court judgement had been around at the time of Jodie's overdose "I don't think it would've gone on for four years".
"All that time me and her children were grieving and we just weren't listened to," she added.
The right to withdraw treatment from vegetative patients began in 1993 with Tony Bland, who had been in a vegetative state since 1989, until the House of Lords agreed that removing "clinically assisted nutrition and hydration" – where a patient is fed through tubes – did not constitute murder.
But they did say referring similar cases to the Court of Protection was best practice.
In Monday's Supreme Court judgement, Lady Black said an agreement between families and doctors was sufficient safeguarding to ensure "public confidence".
But she urged families to apply to court "where there are differences of view" between relatives or medical professionals.
What is a vegetative state?
- A vegetative state is when a person is awake but is showing no signs of awareness. They may open their eyes, wake up and fall asleep at regular intervals and have basic reflexes. They're also able to regulate their heartbeat and breathing without assistance
- A person in a vegetative state doesn't show any meaningful responses, such as following an object with their eyes or responding to voices. They also show no signs of experiencing emotions
- Continuing – or persistent – vegetative state is when this happens for more than four weeks
- Permanent vegetative state is defined as more than six months if caused by a non-traumatic brain injury, or more than 12 months if caused by a traumatic brain injury
- If a person is diagnosed as being in a permanent vegetative state, recovery is extremely unlikely but not impossible
Dr Peter Saunders, director of anti-assisted dying group Care Not Killing, said he was "very concerned and quite disappointed by the Supreme Court ruling" on ending care for vegetative patients, who are "effectively going to be starved and dehydrated to death".
He said it removed an "additional layer" of protection for vulnerable, and "financial concerns" about looking after vegetative patients could mean "decisions may be made for the wrong reasons".
But the charity Compassion in Dying said it would "allow those closest to a person – their loved ones and medical team – to feel supported and empowered to make the right decision for the person, even when it is a difficult one".