Continued high rates of hospital bed occupancy and delayed discharge were contributing to the "relentless" pressure on urgent and emergency care services, NHS Providers said.
The latest situation report from NHS England showed an average of 13,566 beds were occupied with patients medically fit enough for discharge in the 7 days to 22 January.
The number was down from an average of 14,036 the previous week, and from a high of 14,069 in the week ending 8 January.
But Saffron Cordery, the interim chief executive of NHS Providers, warned: "As trust leaders prepare for the biggest NHS staff walkout in less than 2 weeks, they are having to grapple with unsafe levels of bed occupancy."
NHS England said that more than 19 in 20 (95.1%) of beds in adult general and acute hospital wards were occupied.
Ms Cordery said: "All of these stresses are impacting care right through the system, including ambulances, hospital A&Es, mental health, and community services."
The Royal College of Nursing (RCN), which is taking strike action over pay, said the latest figures demonstrated the high workload facing its members. Patricia Marquis, RCN director for England, said: "With thousands of patients still stuck in hospitals in England who are medically fit to be discharged – up from this time a month ago – the pressure on nursing staff and hospitals is extremely high. The effect on staff is evident – with mental health reasons the most cited for nursing staff sickness and almost a quarter of all sick days taken for this reason.
"It has created a perfect storm where patient care is not safe, and staff are under intolerable stress."
Ambulance Handover Delays Improving
There was positive news reported for ambulance handover delays, which saw a further improvement and have now fallen to a new low for this winter. The weekly data showed ambulances transported 77,937 patients to emergency departments in England during the week ending 22 January, up from 72,009 in the previous week. However, despite the rise, 10,170 hours were lost to delays outside hospitals last week compared with 13,921 during the previous 7 days.
In other developments, the number of patients hospitalised with 'flu continued to fall, down to 2034 from a New Year peak of 5441. Norovirus cases in hospitals though ticked up and accounted for 371 adult beds.
Professor Julian Redhead, national clinical director for urgent and emergency care at NHS England, said: "These new figures show that while there have been improvements in ambulance handovers and fewer 'flu patients in hospital, the NHS remains under significant pressure - last week saw more people being taken to A&E by ambulance, bed occupancy is still constrained, and illnesses like 'flu and norovirus are still a very real concern.
"The NHS has done extensive preparation for this winter, including rolling out extra beds, a national falls services, and nationwide 24/7 control centres to track and manage demand, and NHS staff are working flat-out to continue to provide the best care for our patients in the face of ongoing pressures."
NHS Providers called on the Government to publish a "fully funded and costed workforce plan to address staff shortages, as well as help avert more strikes by urgently sitting down with the unions to negotiate pay for this financial year."