David Cameron needs to "get real" and ditch his "surreal obsession" with opening GP surgeries seven days a week, a senior GPs' representative says.
The British Medical Association's Dr Chaand Nagpaul said GP services were already at breaking point.
He will tell a conference that pledges to recruit 5,000 GPs in England would "fail dismally" as doctors were fleeing the profession.
The Department of Health said it was an "overly negative, pessimistic view".
During the election campaign, the Conservatives pledged access to GPs between 08:00 and 20:00, seven days a week, by 2020 in England.
The party also pledged everyone over 75 would get a same-day appointment.
But the proposals have been lambasted in the first major speech by GP leaders since the election.
Dr Nagpaul, the chairman of the BMA's GP committee, , will tell the annual conference of local medical committees: "The government must halt its surreal obsession for practices to open seven days when there aren't the GPs to even cope with current demands.
"It would damage quality care by spreading GPs so thinly, and replace continuity of care with impersonal shift work, and will reduce our availability for older, vulnerable patients."
He will argue no other country was attempting such a plan, while the NHS was pressing ahead with "fewer GPs per head than in Europe, while spending less on health compared to virtually all other comparable nations".
General practice is key to plans to shift more care out of hospitals into the community.
But Dr Nagpaul will use the speech to say demand on services has soared as practices are used as the "backstop for every problem in the NHS and beyond".
There were 40 million more GP appointments annually than five years ago, yet the proportion of NHS funds spent in general practice was falling, he will say.
Dr Nagpaul also pointed to a survey of 15,000 GPs which showed one in three intending to retire and one in five planning to move abroad in the next five years.
Many cited overwhelming workloads.
"It's absolutely pointless promising 5,000 extra GPs within this Parliament if we lose 10,000 GPs retiring in the same period," Dr Nagpaul will say, adding that it would be "utter folly" to dismiss this as simply GPs "scaremongering".
He said there needed to be a national campaign to highlight the pressures on GPs and to encourage patients to use other services such as pharmacies.
He also called for a "complete overhaul" of the non-emergency NHS111 phoneline service as it was "clogging up our appointments".
He said the service referred five million more patients to general practice rather than helping them to care for their conditions at home.
A Department of Health spokesperson said: "This is an overly negative and pessimistic view from the doctors' union.
"Thousands of GPs across the country are already offering patients GP access seven days a week - by next March, a third of the country will be covered.
"We have made it very clear that we will train 5,000 more GPs and have backed the NHS's own plan for the future by investing the £8bn it needs to transform care closer to home."