Doctors' leaders have called for stricter rules on when medics can "conscientiously object" to procedures.
Doctors can conscientiously object to carrying out an abortion
The British Medical Association heavily criticised draft GMC guidance that would allow doctors to opt out of any procedure for ethical reasons.
The plans would give doctors "a licence to discriminate" and should be strictly limited to certain procedures such as abortion, the BMA warned.
But such precise restrictions would not work in practice, the GMC said.
It is currently widely accepted that doctors can "conscientiously object" to certain procedures associated with life and death issues, such as withdrawing life-prolonging treatment, the BMA said.
However, the only absolute right to objection under law is for carrying out an abortion.
The new draft recommendations from the GMC, which have been under consultation, say doctors can object to any procedure for which they have a moral, cultural or religious disagreement.
The BMA said it was "extremely concerned" about how far these objections could extend.
For example, could doctors refuse to refer a lesbian couple for fertility treatment, refuse to carry out a sexual health test on a homosexual or object to treating a Jehovah's Witness who has refused a blood transfusion?
Conscientious objection should be limited to a shortlist of clearly defined procedures, the BMA said.
It added that medical students should not be allowed to opt out of learning about procedures to which they have a conscientious objection.
Some students had raised objections to learning about the moral arguments surrounding abortion and the clinical impact of alcohol, the BMA warned.
Dr Tony Calland, chairman of the BMA's Medical Ethics Committee, said the recommendations seemed to give doctors a mandate to opt out of any procedure they were ethically opposed to.
"Doctors are not there to judge patients but to treat them.
"It would not be appropriate or ethical for a doctor to refuse to help a patient because their own personal beliefs were in conflict with how an individual patient led their life.
"This guidance is confusing and inconsistent and will not benefit doctors or patients."
The GMC said they had received 120 responses to the consultation and would be considering them very carefully.
But a spokesperson said it would be too difficult to put together and enforce a list of certain procedures out of which doctors were allowed to opt.
The guidance was designed to set out doctors' rights to conscientious objection - not to give specifics on individual treatments.
But she added that doctors must not discriminate against patients by denying or delaying treatment for them because of their sexual orientation, gender or race.
Also, the patient must not be left without any treatment at all and should be able to access it elsewhere.
The GMC also said it did not currently limit doctors' right to conscientious objections to carrying out abortions, providing contraceptive services and withdrawing life-prolonging treatment.