A&E performance in England in January dipped to its lowest level since the four-hour target was introduced in 2004.
The percentage of patients treated or admitted within four hours fell to 84.4%, well short of the 95% target and almost a percentage point worse than a year ago. Only two major emergency departments in England hit the 95% target.
A sharp increase in the number of people attending hospital, up 5.6% to 2.112m, meant around 2,000 more people were dealt with within four hours than last January last year, though the total proportion fell.
The decline in performance meant 330,000 people were left waiting longer than they should as hospitals faced huge pressure to find beds.
More than 83,000 patients faced "trolley waits" of a further four hours while a room was found on wards, also the highest figure since current targets were set 15 years ago.
Emergency admissions increased even more significantly than attendances, with 564,000 admissions in the month, up 7.2% on last January.
The decline in key indicators of NHS performance comes after the system appeared to be coping well with the first two months of winter.
With generally mild weather and lower incidence of flu than last year, performance in November and December was better than in 2017, despite a consistent trend of rising attendances.
January saw the first real cold snap of the winter, which may have influenced the decline in performance.
Hospitals have also reported an increase in the number of older, sicker patients requiring admission, with many saying it has been far worse than they forecast.
Figures for January also show that the number of people waiting more than the target of 18 weeks for elective surgery rose to 550,000, the highest level in a decade.
In total 4.15m people are on the waiting list for an operation in England.
Professor John Appleby, chief economist of the Nuffield Trust, said patients faced serious consequences as a result of the decline in performance.
"Today's figures remind us that the NHS is fighting a losing battle in trying to meet its commitments to provide timely health care in the face of the pressure it is under. There is a risk that we lose sight of these problems as Brexit distracts us, or become numbed as we forget the last time targets were met.
"But this situation has a serious impact on hundreds of thousands of patients, and will be demoralising for many staff.
"The proportion of patients spending more than four hours in A&E has risen to 15.6% in January - the highest ever in this set of data. It looks like the gap between the service's capacity and the care we need from it is widening.
"Only two major emergency departments in England met the four hour target and attendances have risen by an astonishing 85,000 compared with January last year, increasing through the winter when they would usually fall.
"Last year there was widespread concern as we saw trolley waits balloon, yet today's figures show an even higher level."
Dr Nick Scriven, of the Society for Acute Medicine, said the NHS was under "severe strain".
He said: "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."