The body which regulates doctors has lodged an appeal against a High Court judgement in a "right to nutrition" case.
Les Burke fears he will starve to death unable to communicate
Les Burke, who has a degenerative brain condition, had brought the case.
He told the hearing in July he feared the existing guidance could allow food and drink to be withdrawn against his wishes when he can no longer speak.
The General Medical Council said it appealed because it wanted clarification on parts of the guidance.
It covers situations where death is not imminent, but doctors believe a patient's condition is so severe, and their prognosis so poor, that artificial nutrition or hydration - giving water - causes more suffering than benefit.
The guidance says that if patients are no longer able to communicate their views, doctors must judge what the patient would want, based on earlier discussions or written statements, and in consultation with patient's relatives.
But Mr Burke, who has cerebellar ataxia and is in a wheelchair, argued that doctors may make a subjective judgement that the patient's quality of life was lower than another person's.
He argued the guidance should be changed so that doctors will have to presume a patient wants to carry on living.
Judge Mr Justice Munby said in his July judgement: "The emphasis throughout ... is on the right of the competent patient to refuse treatment rather than on his right to require treatment.
Announcing the appeal Ruth Evans, chair of the GMC Standards and Ethics Committee said: "The question of whether a life-prolonging treatment should be started, or should be withdrawn, is painful for patients, their family and the doctors to address.
"Some patients will want everything possible to be done, while others will want to avoid too much medical intervention, when they are nearing the end of life.
"Doctors want clear guidance to help ensure individual patients receive the care that's right for them. This is why we are appealing the judgement."
The British Medical Association said it supported the GMC's decision to appeal.
It said that, while it welcomed many aspects of the judgement, there was concern some crucial aspects of law remained unclear, such as when it would be permissible to cease active interventions, such as providing nutrition, for patients nearing the end of life, if they have not made their wishes clear in advance.
The BMA said that, without clear guidance, patients' relatives and doctors could have to go to court more often to obtain rulings in individual cases even, in some cases, where they all agree.
Dr Michael Wilks, chairman of the BMA ethics committee, said: "A decision about when to curtail active interventions for people who are dying or seriously ill is one of the hardest choices that patients, families and doctors make.
"It is absolutely essential for all involved that the law and professional guidance is clear.
"Although in this case, Mr Burke was worried about his active treatment being stopped before he would have wanted, many patients worry about the risks of being kept alive by medical technology beyond the point where they would have chosen non-invasive palliative care.
"We hope that the GMC appeal will clarify ambiguities in the judgement without losing many of the helpful points in it about listening to patients' views."
Mr Burke, 44, from Lancaster told BBC News Online "I'm a bit taken aback, I thought they had more common sense.
"The ruling was pretty clear," said Mr Burke who added he would challenge any attempt to reverse or water down the judgement.