Almost a third of NHS hospitals in England hit maximum capacity at least once this winter, figures show.
General and emergency wards were on average 93.5% full between December 3 2018 and March 3, according to NHS England data.
A total of 41 out of 134 acute NHS trusts (31%) reported bed occupancy rates of 100% on at least one day during this period.
Three-quarters of trusts (75%) had average bed occupancy rates of 92% or more across winter.
NHS Improvement suggests A&E performance begins to deteriorate when bed occupancy exceeds 92%, while the British Medical Association (BMA) recommends 85% capacity as a safe upper limit.
Dr Nick Scriven, president of the Society for Acute Medicine, said hospitals are managing capacity levels “on shoestring budgets and with depleting workforces”.
He added: “For those working at the sharp end of the NHS, the figures are no shock.
“Our hospitals have been on a knife-edge for more than 18 months now, with staff run ragged trying to keep them safe.
“We have seen a year-on-year reduction in the beds available to care for acutely unwell people in hospitals, despite an ever-increasing need from an ageing population.”
An average of 3,162 escalation beds – which are temporarily set up to cope with demand – were in use in England’s hospitals every day during this period, the data shows.
Healthcare workers union Unison said cuts to social care budgets have led to a growing problem of long-stay patients, which affects whether hospitals can accommodate urgent admissions.
On a typical day, more than 40,000 patients had been in hospital for a week or more, and an average of 15,300 had been in hospital for at least three weeks.
Sara Gorton, Unison head of health, said: “Although this winter’s weather has been relatively mild, there has been no let-up in the pressures facing patients and staff, laying bare serious underlying problems.
“Meanwhile, ambulance crews are tied up with handover delays rather than being where they and the public want them to be – responding to 999 calls and saving lives.”
More than 1.2 million people were taken by ambulances to acute hospital trusts over the winter months, according to NHS England data.
Delays left 146,000 patients waiting 30 minutes or more before they could be transferred to the care of A&E staff, and of those nearly 30,000 waited for more than an hour before they were admitted.
An NHS spokesman said: “NHS staff across the country have been working incredibly hard throughout winter to provide the best care for patients.
“Despite significant increases in demand, almost a quarter of a million more people have been seen and treated within four hours in A&E this winter compared to last year.
“Ambulance services are responding to life-threatening calls faster, with fewer ambulance handover delays at A&E, and significantly more people have got the support they needed to avoid a long stay in hospital.
“Meanwhile tens of thousands more people are benefiting from timely tests or treatment, including for cancer and mental health.”