Chief executives warned that bed occupancy rates are already approaching 99 per cent in some parts of the country, compared with recommended levels of 85 per cent.
A report by NHS Providers warns of “severe pressures” and “substantial risks” to patients amid fears that Britain could be hit by the worst flu strain in decades.
It comes amid growing tensions between health officials and Government over the funding boost awarded to the NHS in the recent budget.
Simon Stevens, head of the NHS, last week suggested that waiting list targets would have to be sacrificed in order that other services, such as A&E, cancer and mental health, could be protected.
Hours later, Jeremy Hunt last week insisted waiting times must still be hit.
It came after the Chancellor awarded the NHS £2.8bn over three years, including £1.6bn next year - less than the £4bn which NHS officials had asked for.
Latest published figures show general and acute bed occupancy levels averaging almost 89 per cent across the country, during the summer - and exceeding 99 per cent in some parts of the country.
NHS Providers said there was now “very little give” in the system, warning of shortages of paramedics, GPs, and A&E consultants and nurses.
The body, which represents NHS hospitals, also said that an extra £335m awarded in the budget for winter pressures had come too late for best use to be made of it.
Hospital trust chief executives said much of the funding would be spent on agency staff, because it was too late to be used for longer-term planning.
Chris Hopson, chief executive of NHS Providers, said: “Winter always presents a big challenge to the NHS.
“Last year the pressures were intolerable. Services were stretched up to, and in some places beyond, breaking point.
“This time preparations have never been more thorough,” he said.
“But we have to recognise we are not where we would want to be as we head into winter.
“The NHS is already under severe pressure, and while the additional funding in the recent Budget is welcome, it has come very late to be used to maximum effect.
“We can not say with certainty how tough this winter will be, but the likelihood is that services will be sorely tested,” he said. Later today the NHS will publish the first set of weekly figures showing how hospitals are coping as winter sets in.
Latest published bed occupancy levels, general and acute beds, highest five
United Lincolnshire Hospitals Nhs Trust 99.3%
Bedford Hospital Nhs Trust 98.9%
Milton Keynes University Hospital Nhs Foundation Trust 98.8%
Kettering General Hospital Nhs Foundation Trust 98.8%
University Hospitals Birmingham Nhs Foundation Trust 97.5%
NHS England, quarter ending September 2017