Thousands of patients could be left without a GP if more is not done to address a "workforce crisis" among family doctors, experts have said.
A large number of family doctors are nearing retirement age and unless "drastic action" is taken to find medics to replace them, patients could be left without a GP or be forced to travel miles to their nearest practice, t he Royal College of General Practitioners (RCGP) said.
The college said that at around 600 practices across the UK, including 543 in England, 90% of GPs were over the age of 60.
Dr Maureen Baker, chairwoman of the RCGP, will tell the college's annual conference in Liverpool that the number of people entering the profession was falling short of the numbers leaving it.
As well as retiring, family doctors were leaving Britain to work abroad or pursuing entirely different careers, she will say.
The RCGP said that the number of unfilled GP posts had quadrupled in the last three years from 2.1% in 2010 to 7.9% in 2013.
Meanwhile, estimates suggest that the applications for GP training have dropped by around 15%.
To compete with rising demand, there needs to be around 40,000 GPs in England but there are only around 32,000, it added.
Dr Baker will call for the budget for general practice to be increased to help meet this workforce gap.
Comparing general practice to the "walls of a dam" that prevents the rest of the NHS being flooded, Dr Baker will say: "So far, much of the damage to the dam wall has been hidden from the public - they see the flooding downstream in accident and emergency departments and in hospital pressures, but they haven't been aware that GPs, nurses and practice teams have been absorbing that pressure by trying to do more and more with less and less.
"But if we let that situation continue, we will see whole chunks of the dam fall apart when practices have to shut their doors.
"Every practice closed is a loss to a local community.
"Not only do patients lose out, but it piles more pressure on neighbouring practices, swelling patient lists already bursting at the seams.
"Today, I can reveal new estimates from the College that 543 practices in England are at risk of closure if something isn't done.
"There are practices that have over 90% of GPs over the age of 60, when the average retirement age of GPs is 59 - this is shocking.
"With a growing, ageing population, not to mention a baby boom, we need to increase capacity in general practice, not take it away.
"If this was a business, it would be expanding to meet demand - not shutting down services and closing branches."
She will add: "Most worryingly, in the face of relentless workload pressures and constant attacks from the media, we are not attracting enough new doctors and nurses into general practice, or doing enough to retain the highly skilled workforce we have.
"All of these developments result in further weakening of the dam.
"Colleagues, the wall of the dam - the service of general practice - is under huge pressure and unless urgent action is taken to repair and restore the dam, it could burst, with terrible consequences for our patients in general practice and indeed for the whole of the NHS."
Commenting on the speech, shadow health secretary Andy Burnham said: "This dose of reality from the Royal College of GPs completely undermines David Cameron's party conference rhetoric. The truth is that this Prime Minister has presided over a crisis in general practice and collapse in GP morale.
"People are already struggling to get GP appointments and these figures suggest things are about to get even worse, not better, if hundreds of surgeries close for good.
"This is a problem of the Prime Minister's own making and he must urgently produce a convincing plan to sort it out. Unfunded, unrealistic party conference promises are not good enough. Patients must not be left travelling miles to see their GP or waiting weeks for an appointment.
"Labour's pledged an extra £2.5 billion for the NHS - on top of the Tory plans - to pay for 8,000 more GPs and to guarantee appointments within 48 hours. David Cameron has failed to match this investment."
A Department of Health spokesman said: "We know GPs are under pressure, which is why we're increasing trainees so that GP numbers continue to grow faster than the population and will train 10,000 more primary and community health and care staff by 2020, including 5,000 more GPs.
"There are already 1,000 more GPs than in 2010 and we're making it easier for GPs to return to the profession."
Dr Chaand Nagpaul, chairman of the BMA's GP committee, said: "The nation depends on their GP service, with about one million patients visiting their surgery daily.
"The pressure on GP practices has rocketed, with a growing older population with multiple needs and increasing care moving out of hospital. This has not been matched by an increase in GPs and practice staff, leaving the service under unprecedented strain.
"It is alarming that surgeries are under threat from cuts to funding, difficulties in recruiting doctors and GPs retiring early from unmanageable workload pressures, denying patients the vital care they need.
"The BMA has called for urgent action to reverse this with a plan of immediate measures to support and develop GP practices at this critical time."