Hospitals have run out of beds and can no longer cope, bosses are warning.
Reports have emerged since Christmas of patients being left for hours on trolleys in corridors and stuck in ambulances as A&E teams struggle.
Chris Hopson, of NHS Providers, said hospitals were now unsafe and over-crowded and the government should rethink its policies.
But ministers say plans are in place to help the health service cope, despite mounting evidence of growing pressures.
Last week the BBC reported that thousands of ambulances had been left stuck outside Accident and Emergency departments over the winter because there were no staff available to receive their patients.
Inside A&E patients have complained of being left in waiting rooms and corridors for hours as staff struggle to free up beds.
Latest figures show that hospitals in England are well above the 85% bed occupancy rate considered to be safe to ensure beds are ready when they are needed.
'There was no dignity – it was chaos'
Rosie Dawson, 37, is just one of the patients who has been caught up in the problems.
She was taken to Torbay General A&E on 3 January with a gynaecological problem which had left her with severe pain and bleeding.
She said it was chaos, with trolleys everywhere, staff running up and down corridors and queues of ambulances outside.
Staff could not find a private area for her to be assessed so she ended up being examined in front of other patients.
"There was no dignity. It was degrading," she said. "I couldn't fault the staff, there was nothing they could do. It was chaos."
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Mr Hopson said it was clear from the evidence that was emerging that the service was "over-stretched and staff run-ragged", with many places having no free beds at all.
"Too often, in too many places, standards of care are compromised and patients' safety put at risk.
"In the past mild weather and low flu rates helped us scrape by. Maybe if we had been lucky again this year we could, just about, have coped.
"But it has not turned out that way.
"Flu is rising, there is more respiratory illness and the cold weather is taking its toll."
Fresh figures on NHS performance in England, released on Thursday, covering the end of last year and beginning of 2018, are expected to show deteriorating performance.
Figures have already been published for Scotland for the end of 2017. They showed record levels of long waits in A&E.
Mr Hopson, who pointed out even before the recent struggles that the NHS was already missing its key targets for A&E, cancer and planned operations across the UK, said the current situation should act as a "watershed moment" and prompt the government to find a long-term funding solution.
Since 2010 the budget has been rising at about 1% a year on average whereas traditionally the NHS got over 4%.
A spokesman for the Department of Health and Social Care in England acknowledged there were problems.
He said: "We know there is a great deal of pressure in A&E departments and that flu rates are going up, and we are grateful to all NHS staff for their incredible work in challenging circumstances."
But he added that plans were in place to help, including extra money for council-run care services so people could be moved out of hospital more quickly and the single biggest expansion in doctoring training places in the history of the NHS – 25% in the coming years.