GPs may feel forced to quit the NHS if the government pushes through its plans for them to work extra hours.
GPs pay has risen since a new contract came in
The option was laid out in a set of briefing slides used by the British Medical Association (BMA) in its talks with doctors.
But the BMA insisted it was not asking or encouraging GPs to leave the NHS - only that quitting was a possibility.
It comes after the government threatened to impose a tougher contract if GPs do not agree to longer hours.
The BMA, the doctors' trade union, has agreed to two extra hours a week for every 6,000 patients.
However, the government wants an extra three hours, and has stepped up pressure on GPs to accept.
There is also disagreement about where the money to pay for the work should come from.
Gordon Brown has made extending GP hours a priority, although a survey of 2m patients earlier this year showed 84% were happy with opening.
Doctors leaders also warned the government has a "clear agenda" to introduce private firms in the NHS.
It has been suggested that in areas where there is demand for longer hours, the government would be prepared to see companies move into the primary care market if NHS doctors refuse to respond.
BMA chair, Dr Hamish Meldrum, said GPs would not want to take any action that would harm patients.
"I want to make it clear it's not BMA policy, it's not BMA desire that GPs end up resigning from the NHS."
He said GPs were angry that the agreement over extended hours had been changed at the "11th hour" and that there was not a "level playing field" with private providers when bidding for local contracts.
"GPs want to see the NHS preserved and very much the preferred option is to try to seek resolution to the current problems."
Dr Laurence Buckman, chairman of the BMA's GPs committee, said GPs care passionately about the NHS and have "great fears" about the introduction of private firms into primary care.
"We are naturally looking at all the options if faced with an imposition by government, including advising GPs of the consequences of leaving the NHS," he said.
The briefing slide said there would be "huge political impact" from such a move.
But it said private GPs could charge patients between £20 and £25 per consultation, with additional fees for other work.
Health minister Ben Bradshaw said the government was revising the GP contract to take account of demands for extra hours.
"We hope that frontline doctors will support our efforts in the interest of their patients, rather than following the sensationalist plan to leave the profession proposed by the BMA."