The Supreme Court is due to rule on whether it should be easier to withdraw food and liquid to allow people in long-term vegetative states to die.
Thousands of people are believed to be in this state, although precise figures are not available.
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The case concerns a banker in his fifties who suffered a heart attack and severe brain damage.
He has since died, but the case has continued to allow the court to make a ruling.
After his heart attack last year, the banker, known as Mr Y, was left in an unresponsive state with no prospect of improvement.
His family and his doctors agreed it would be in his best interests to withdraw his feeding tube and allow him to die.
For the last 25 years, such cases have been decided by the Court of Protection, which makes decisions on welfare issues for people who lack the mental capacity to do so themselves.
The NHS Trust asked the High Court to declare that this process was not necessary where the doctors and the family all believe it is in the patient's best interests.
Applications to the Court of Protection can take years and cost thousands of pounds.
The judge agreed, but the official solicitor appealed on behalf of Mr Y.
Weakens the law
A Supreme Court ruling could offer certainty to the families of thousands of people believed to be in similar conditions, some who have been in a vegetative state for 20 years.
Jean Simpson, whose daughter Jodie was in a vegetative state for four years after a drug overdose, said: "We all went through nearly four years of hell and constant grieving. Nobody should have to suffer that."
But anti-euthanasia groups have warned it could weaken protections for vulnerable patients.
Peter Saunders, campaign director of Care Not Killing, said: "Taking these decisions away from the Court of Protection removes an important layer of legislative scrutiny and accountability and effectively weakens the law."
Decisions on a patient's future could be influenced by those with "ideological" or financial interests, he said.
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