The Chair of the British Medical Association says that claims that doctors can provide a "24 hour a day 365 days a year service are ridiculous".
Dr Mark Porter told delegates that the health service can barely afford its current model, and that plans for doctors to do more would only add to that burden.
"Like many of you I work nights and weekends as well, at times when much of the private sector is fast asleep and ministers are tucked up soundly in their beds" he said.
"We all want urgent care at weekends and evenings to be of the same high standards as patients can expect on weekdays.
"But the calls we sometimes hear for a Tesco NHS, full service, 24/7, are just ridiculous when the health service can barely afford its current model.
More top news
The coup shows a split in the cabinet between ministers who want to stop a no-deal Brexit, and those who want to prevent a referendum.
Speculation that the PM will be ousted from Downing Street has reached fever pitch.
Newly-released papers show a young David Cameron condemning ‘Euro red tape’ while working as a researcher for the Conservative Party in 1988