A judge's ruling on the rights of terminally-ill patients could put doctors in an "impossibly difficult" position, the GMC has told a court.
Leslie Burke won the original case
Leslie Burke who has a degenerative brain condition, won a landmark ruling last year to stop doctors withdrawing food and drink when he cannot speak.
The 45-year-old feared General Medical Council rules on artificial nutrition may allow his wishes to be over-ruled.
But the GMC has taken it to the Court of Appeal as it wants clearer guidance.
Artificial nutrition is classed as a form of treatment by the GMC and, therefore, last July's ruling, which was hailed as a breakthrough for the rights of terminally-ill patients, raised questions about medical intervention.
Philip Havers, QC, representing the GMC, said the original ruling fundamentally altered the nature of doctor / patient relationships and was not in the best interests of the patient.
He said doctors would have to provide treatment which they knew would be of no benefit or could even be harmful.
Mr Havers said such a conclusion "puts the doctor in an impossibly difficult position, for a doctor should never be required to provide a particular form of treatment to a patient which he does not consider clinically appropriate."
He added there could come a time - many years ahead - when Mr Burke may find himself in circumstances where it would be better for artificial nutrition to be withdrawn.
And he said: "There is no reason to suppose that he would not be treated as he would wish to be treated.
"It is undesirable to make such a declaration because it will tie the hands of the doctors and the court as to what treatment can and cannot be provided."
Mr Burke, from Lancaster, has cerebellar ataxia - an umbrella term for nervous system disorders that cause lack of co-ordination, and can lead to difficulty walking and confine sufferers to a wheelchair.
It can cause slurred speech and problems with swallowing, and lead to loss of sight and hearing - but mental faculties are not affected.
In the original case, he argued the GMC advice, which gives doctors in cases such as Mr Burke's the ultimate say on what treatment to give a patient in the final stages, was an infringement of his human rights.
Mr Justice Munby sided with Mr Burke and said that if a patient is competent - or, if incompetent, has made an advance request for treatment - doctors have a duty to provide artificial nutrition or hydration.
Since the ruling, the GMC has told doctors to adhere to the judge's verdict.
Ahead of the start of the case on Monday, Mr Burke said if doctors did withdraw food it could take days for him to die, during which time he would be aware of what was happening.
"That is not a dignified way to go."
Commenting on the case, Professor Irene Higginson, who specialises in palliative care and gave evidence on behalf of the GMC in the original hearing, said there were cases where giving food and water artificially could do more harm than good.
And Dr Vivienne Nathanson, the British Medical Association's head of science and ethics, said: "The original case has left us with great uncertainty as to who can make decisions and in what way."
But Roger Goss, co-director of Patient Concern, which is also represented at the hearing, said it was wrong to class artificial nutrition as treatment.
"Feeding someone through a tube because they cannot swallow is not treatment, and should not be treated as such. Doctors should not have the right to take this away against a patient's wishes."
The hearing, expected to last three days, has been adjourned until Wednesday.