The NHS is preparing to face the 'worst winter it has ever endured' with more than a million patients expected to wait more than four hours in A&E over the cold season.
Almost 350,000 patients could also end up stranded on trolleys in corridors as they wait for treatment, the British Medical Association (BMA) has warned.
The union said harsh weather conditions, the potential of a severe flu outbreak and the uncertainty of Brexit could cripple the health service this winter.
Experts say at least another 10,000 beds are needed to give hospitals in England and Wales a chance over the coming months.
The BMA's warning comes after recent years have seen staff overloaded by people wanting hospital treatment during the cold spell.
The British Medical Association predicts more than a million patients could be forced to wait for more than four hours in A&E over the winter
BMA chairman Dr Chaand Nagpaul said: 'Enough is enough. Right across the health service, trusts and GP practices will be bracing themselves for what looks set to be the worst winter the NHS has ever endured.
'Patients should not fear needing hospital care or being able to see their GP, nor should they have to accept that they may spend hours on a trolley in an emergency department waiting to be treated.'
He made the comments as the association publishes its election manifesto to lay out demands for the next Government to address.
The warning comes as part of the BMA's worst-case scenario, which drew on previous NHS data.
Interviews for the union's Doctor magazine saw one medic describe how patients have suffered cardiac arrests in corridors while they waited for treatment.
The unnamed Midlands emergency medicine consultant said the summer had been 'absolutely brutal'.
They added: 'It's really corrosive for staff because they are coming into work with patients lining the corridor.
'We have been putting patients onto bits of corridor we have never used before - we are actually expanding into new areas.
'There have been cardiac arrests on the corridor in a number of places. We are completely under the cosh and I don't think there's an emergency department in England or Wales that thinks that patient care isn't going to suffer over winter.'
Another south-west specialty trainee in emergency medicine said conditions were 'not safe'.
The NHS is on a collision course as it heads into winter with waiting lists for routine operations at an all-time high, experts have warned.
A damning report showed more than 4.41million patients were stuck on waiting lists in England in August - up by 250,000 from last year.
And 662,053 people have waited more than 18 weeks for routine treatments, such as joint replacements - the highest since records began.
Health leaders condemned the figures and said they show the NHS could face its worst winter ever with Brexit, harsh weather conditions and flu on the horizon.
NHS bosses said trusts up and down the country are working ‘incredibly hard’ to prepare for the winter and make sure patients are kept safe.
But the Royal College of Nursing fears more and more patients are going to be treated in corridors as pressure gets piled on the health service.
And the Royal College of Surgeons warned the upcoming winter pressure, Brexit and the NHS pension crisis will create a ‘perfect storm’ for hospitals this winter.
Experts called for 'swift and far-reaching' Government action to get the NHS braced for winter.
The comments came after Nottingham University Hospitals Trust (NUHT) declared a 'critical incident' this week due to pressures on its emergency services.
Simon Walsh, NUHT's emergency medicine lead, said the union's analysis showed 10,000 more beds were desperately needed.
He added: 'It is notable that even the chief executive of NHS England, Simon Stevens, has recently acknowledged that we need to go into winter with