There has been a sharp rise in "trolley waits" - the length of time people wait for a hospital bed after being admitted in an emergency, figures show.
Data analysed by the BBC shows 473,453 patients waited more than four hours between October 2015 and September 2016 - almost a five-fold increase since 2010/11.
The figure represents 11% of the 4.2 million patients admitted over the period. More than 1,400 faced delays of more than 12 hours.
In 2010/11, there were 97,559 trolley waits - although NHS England said a small fraction of the rise could be attributed to a change in the way the waits were measured last December.
While the waits are known as trolley waits, some patients wait in side rooms, seats in the A&E department and spare cubicles before being admitted to a ward.
The BBC also reported that three-quarters of hospitals are reporting bed shortages. Bed occupancy is not meant to exceed 85%, to give staff time to clean beds, keep infections low and ensure patients who need beds can be found them quickly.
But 130 out of 179 hospital trusts are reporting rates exceeding this for general use beds.
Siva Anandaciva, of NHS Providers, which represents hospitals, said: "These figures are deeply worrying. We are heading into winter in a more fragile state than I have seen in the past 10 years or so.
"Even the historically top-performing trusts are being challenged, which shows that this is an issue affecting all parts of health.
"No-one wants to see people waiting in corridors, side rooms and emergency bays when they should be admitted to a hospital bed. These patients are still under the care of doctors and nurses, of course, but it is not ideal for them and we know overcrowding leads to worse outcomes."
Dr Chris Moulton, of the Royal College of Emergency Medicine, told the BBC: "Patients who are delayed like this are still being monitored by staff. But we know that the overcrowding we are seeing is dangerous. It leads to worse outcomes for patients - higher infection rates, patients ending up on the wrong wards, and generally a negative experience."
Dr Moulton believes there are too few beds. There are just over 100,000 general beds in England - a fall of 40,000 in the past 20 years.
"We simply don't have enough," he said. "If you compare us to other European countries we are really short and the demands being placed on the health service means we are now struggling to cope."