Patients are being put at increased risk of infection because hospitals are so full, a report warns.
The NHS is having to open thousands of extra beds - the equivalent of at least an extra five-and-a-half new hospitals on any given day - to cope with demand , a new briefing from the Nuffield Trust found.
On the single busiest day last winter, an extra 4,390 beds had to be opened - equivalent to more than seven extra hospitals in one day.
On average, more than 95% of beds in England's hospitals were occupied every day last winter and the Nuffield Trust said a worrying picture was emerging this winter.
Research shows that once hospital bed occupancy rates rise above 85% to 90%, there is an increased risk of infection. Bed occupancy should be kept around 85% to give staff time to clean beds properly and ensure they can quickly find beds for patients who need them.
The new report says high bed occupancy means it gets " harder and harder" to find beds for emergency patients who need to be admitted from A&E. This has a knock-on effect of making hospitals miss the target of ensuring 95% of patients attending A&E are treated, admitted or discharged within four hours.
Pressure on beds can also make patients' experience of hospital "unpleasant and disruptive", the report said. This is because patients are moved around to accommodate other patients or are put on inappropriate wards.
The report also says hospitals need some "slack in the system" to be able to cope with unexpected demand, such as from a flu or norovirus outbreak.
One of the issues is that thousands of beds across the NHS have people in them who are medically fit to leave but who cannot go home because care packages are not in place.
Looking at last winter, the Nuffield Trust found that on Monday January 25, one in seven trusts had all their acute beds full, and almost four out of 10 had bed occupancy levels of over 98%. This was after opening nearly 4,200 extra beds that day.
Nine trusts were full every day during that week, with 100% of beds and extra beds at capacity.
The trusts were University Hospitals Birmingham NHS Foundation Trust, Bolton NHS Foundation Trust, Northampton General Hospital NHS Trust, North Middlesex University Hospital NHS Trust, Isle of Wight NHS Trust, Great Western Hospitals NHS Foundation Trust in Swindon, Basildon and Thurrock University Hospitals NHS Foundation Trust, Kettering General Hospital NHS Trust and Epsom and St Helier University Hospitals NHS Trust.
The following Monday, 72% of all trusts had bed occupancy levels exceeding 95%.
Professor John Appleby, chief economist at the Nuffield Trust, said: "Our analysis shows just how acute the pressure on beds was last winter, with around 95% of the beds in all hospitals in England occupied every day.
"With such high levels of bed occupancy linked to higher infection rates and longer waits in A&E, these pressures pose a real threat to the smooth running of hospitals and, ultimately, to patient safety.
"What's more, the NHS is going into this winter in an even worse position than it was a year ago, with record deficits, worse performance against the A&E target, far more trolley waits, record delayed discharges from hospital, and fewer people getting the help they need with social care.
"When you add into that mix the sort of intense pressure on beds we've demonstrated hospitals experienced last winter, patients' care is bound to suffer."
The report comes as the British Heart Foundation (BHF) warns that cold spells caused nearly 6,000 extra deaths last winter from heart attack and strokes.
It said a three-day cold spell doubles the risk of heart attack and stroke compared with shorter cold snaps, with drinkers and smokers most affected.
Associate medical director, Dr Mike Knapton, said: "The strong spike in deaths from heart attacks and strokes during prolonged cold spells highlights the need for research into the factors that specifically increase people's risk in the winter and how people can reduce it."
He said people needed to get the flu jab, adding: "People with chronic heart disease are around 11 times more likely to die if they develop flu compared to healthy individuals."
Dr Mark Holland, president of the Society for Acute Medicine, said: "Just over a year ago we suggested that the NHS in England faced a 'perfect' storm and we are now approaching the eye of it.
"Come the first week in January as the social care crisis continues to bite at such a critical time of the year, we will have patients spread around our hospitals, often in inappropriate specialty beds.
"This is bad for the patients in those beds and for the system as a whole as we cannot admit routine cases. The strain on social services will be even greater as we struggle to clear the backlog."
Shadow health secretary Jonathan Ashworth said: "Labour has been warning for some time that the failure to tackle the crisis in social care has huge knock-on implications for bed availability and patient care.
"It is therefore all the more astonishing that Theresa May continues to bury her head in the sand refusing to find a single penny of extra cash for the NHS or social care this winter."
An NHS England spokesman said: "This report looking back to last year points to the steps the NHS takes each winter to maximise bed availability, and plans are well in hand for this winter too."