Dr Chaand Nagpaul of the British Medical Association said a national shortage of GPs was already a "real challenge" without the pressure to work additional hours.
Speaking after Prime Minister David Cameron urged longer opening hours for GPs' surgeries, Dr Nagpaul said they are "currently unable to cope with the increased demands of care".
He told ITV News: "These additional hours amount to around a 58% increase on current levels and we really don't have the GPs to meet current demands, let alone an addition 58% increase in hours."
The NHS does not have "as many" doctors per hospital bed as we need to lower the death rate, a former head of the British Medical Association has said.
Dr Brian Jarman echoed comments made by a Royal College of Physicians report that patients were more likely to die at the weekend because there were not enough staff.
He added: "Complaints by patients...at the moment don't investigate 99.7% of them. What industry would do that?"
Criticism of the NHS by Health Secretary Jeremy Hunt is having a "very, very bad effect" on morale, shadow health minister Lord Hunt of Kings Heath said today:
Hardly a day goes by without the Secretary of State taking the opportunity to attack various aspects of the National Health Service.
This is having a very, very bad effect on morale within the NHS and Mr Hunt should desist. A period of silence from him would be very welcome.
Health Secretary Jeremy Hunt has told reporters that the British Medical Association's decision to pass a vote of no confidence in him is "sadly predictable".
Jeremy Hunt has responded to the vote of no confidence given to him by the British Medical Association by accusing the union of being "sadly predictable".
"The BMA is the doctor's union and I'm afraid its sadly predictable that when someone speaks out about problems in the NHS they would react like that" he said.
"But what I would say its not just me who want's to improve patient safety on the NHS, every week, it's horrible to say but we operate on the wrong part of someone's body in some part of the NHS".
"I gave a difficult speech about that last week saying we'd got to do better and i think most doctors would support me. I think it's my job to speak up for the patients without fear or favour and that's what I will continue to do".
Dr Paul Flynn the chairman of the BMA Consultants Committee has told ITV News that doctors felt like they were scape goats for "problems which are in the Health Secretary's remit".
"We are always open to working together we want to form a common ground with the Health Secretary. But if the Health Secretary is blaming doctors for the problems then that is very difficult".
A spokeswoman for the Department of Health has said the Health Secretary Jeremy Hunt "demands the best possible care for patients", after doctors at the annual British Medical Association Conference passed a vote of no confidence in him.
"Following the findings of the Francis Inquiry and other recent reports, it is clear that the culture of the NHS needs to change and it is disappointing that the BMA union still doesn't accept that" she said.
Dr Mark Porter, Chairman of the British Medical Association conference has accused the government of consistently acting "against the best interests of patients", after Doctors approved a vote of no confidence in the Health Secretary Jeremy Hunt.
The traditional thing to say is that we should focus on the politics and not personalities. We have policy from 2012 that we have no confidence in the then Secretary of State. I recall sitting out in the audience at that time and being told 'be careful about whom you have confidence in because the guy who follows may not be any better', and of course we've seen exactly that.This present Coalition Government goes out of its way to act against the interests of patients.
Delegates at the British Medical Conference have reacted to their vote of no confidence in the Health Secretary Jeremy Hunt.
Consultant Jacky Davis told the conference that "Mr Hunt is about as popular as George Osborne at the Olympic medal ceremony".
"Mr Hunt is ready to blame anyone and everyone rather than the government" she continued.
"His new cunning plan is to cut back the service, the staff and then blame the NHS and say it can only be saved by the private sector".
The British Medical Association has passed a vote of no confidence in the Health Secretary Jeremy Hunt.