Accident and emergency departments suffered their second-worst month on record in January, figures show.
On a more positive note, the performance marked against the four-hour waiting target saw a slight improvement on December.
In total 85.3 per cent of patients were seen, treated or discharged within the allotted four slot in January. This is considerably short of the 95 per cent target, but still an improvement on the 85.1 per cent from December.
More than 1,000 patients had to wait more than 12 hours to be seen - more than double the previous month.
Dr Nick Scriven, president of the Society for Acute Medicine, said the figures demonstrate how the NHS does "not currently have a sustainable model" to cope during the busy winter months when illnesses such as flu and norovoris are more prevalent.
"The last six weeks has seen the acute services of the NHS under a sustained period of stress due to 'normal' winter pressures along with a surge in influenza," he added.
"Neither of these were unpredictable but both have combined to cause the issues that have been widely reported across the country.
"Last year we coined the phrase 'eternal winter', but the last month and a half has shown an even steeper decline in performance as demonstrated by all the data available - particularly around ambulance delays, the four-hour emergency target and bed occupancy both in acute beds and critical care."
An NHS England spokesman said: "Despite the worst flu season in seven years, A&E performance improved this month.
"It was better than both the month before, and was better too than the same time last winter.
"This was partly helped by the fact that NHS-related delayed transfers of care fell to their lowest in four years freeing up beds for patients needing emergency hospitalisation."