A&E waits in England have reached their worst level since the four-hour target was introduced in 2004.
The deterioration in performance came after hospitals appeared to be coping well in the early part of winter.
During January, just 84.4% of patients were treated or admitted in four hours – well below the 95% threshold.
It means nearly 330,000 patients waited longer than they should with hospitals reporting significant problems finding beds for those needing to be kept in.
More than 80,000 patients were kept waiting an extra four hours or more to be transferred to a ward after their wait in A&E.
These are known as trolley waits since patients are left in temporary waiting areas while a bed is found.
All this comes despite relatively low levels of flu.
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Dr Nick Scriven, of the Society for Acute Medicine, said it was clear the NHS was under “severe strain”.
He said hospitals had seen significant over-crowding with many intensive care units completely full.
He said this had had a knock-on effect on ambulances which were being delayed dropping off patients at A&E.
“Although there is less minor illness associated with flu this year, there are more severely ill people than last year which is putting an even bigger strain on the critical care facilities in our hospitals.
“Any NHS worker will tell you that the stresses and strains are very real and ongoing with no let up in sight.”