Just 85.3% of patients were seen at accident-and-emergency departments within the waiting-time target of four hours in January - the second-worst month on record.
NHS England said the "worst flu season in years" had put a strain on services but the result was an improvement on December and January last year.
These were the joint worst months since records began.
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."
NHS England said more than 1.7 million patients were seen within four hours last month, an increase of 5.72% on the daily average for the same month last year.
There were 36 cases of ambulances being diverted to other A&E departments last week, compared with 43 in the previous week.
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."