It could mean that Leslie Burke, who suffers from a degenerative disease that will eventually rob him of the ability to communicate, is left to starve to death against his wishes.
Last year Mr Burke, 45, won a judgement that doctors must, if a patient is mentally competent or has made an advance request before being incapacitated, provide them with food or water to keep them alive.
But the General Medical Council (GMC) appealed, saying that it would leave doctors in an impossible position. There would be a possibility of doctors being forced to give artificial nutrition or hydration (ANH) even though to do so was not in the best interests of the patient, it said.
On Thursday, the GMC won its appeal.
Although the appeal judgement suggests that any doctors cannot deliberately deny nutrition to bring the life of a patient to an end, physicians can still do so if they believe it is in their patient's best interests.
Although Mr Burke was refused leave to appeal to the House of Lords, he is still able to petition the house for a hearing.