The NHS is under "severe pressure" as it faces potentially the worst flu season in two decades, health service experts have warned.
NHS Providers said the health service in England would be "sorely tested" during the colder months, when there was traditionally a significant rise in demand for services due to flu, norovirus, and respiratory conditions.
While winter planning in the health service in England was "extensive and more effective than ever before", the NHS was "not where it would want to be" heading into winter, according to the organisation, which represents health service acute, ambulance, community and mental health services.
A new briefing paper from NHS Providers on how NHS Trusts were preparing for winter stated that although there had been improvements in some performance areas, there were still "risks" in the system.
These included a more virulent flu strain, problems with capacity, workforce shortages and pressurised finances.
Bed occupancy rates were running above the recommended safe levels with 87.1% of beds already full. This meant there was already "very little give in the system", the briefing paper stated.
The Society for Acute Medicine said the NHS was 1,400 beds "short" of what it needed this winter.
Meanwhile the NHS Providers briefing paper warned that this year's flu strain was "potentially the worst we have seen in two decades".
The NHS is anticipating a similar flu outbreak to that seen during the flu season in the Southern Hemisphere, where hospitals were forced to close their doors to new patients and people faced long waiting times.
The briefing paper stated that the NHS had a long history of advance planning to deal with flu and had "significantly increased its focus" this year, including the use of incentive payments for hospitals which had 75% of staff vaccinated.
Other problems included shortages of key staff groups including paramedics, GPs and A & E consultants and nurses.
It outlined various steps that had been taken both at local hospitals and by NHS England and the Government to mitigate winter problems, including more money for winter pressures from the budget and contingency planning by NHS England.
But 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.
"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.
"We must hope the considerable efforts to curb the impact of flu are successful."
Commenting, Dr Nick Scriven, president of the Society for Acute Medicine - which represents hospital doctors who look after patients admitted as an emergency who do not need surgery - said: "While staff on the frontline will, as always, pull out all the stops to provide safe care, the fact remains the NHS is 1,400 beds short of what it needs this winter.
"The fear is that we have not faced an infection crisis over winter for several years and if the Australasian experience is repeated here the system will be swamped as never before.
"There is a real sense of foreboding that this may be the winter that finally breaks the backbone of the service."
Sara Gorton, head of health at the union Unison, added: "This alarming report shows the amount of strain the NHS is under even before winter pressures have kicked in.
"A lack of beds, staff shortages and the funding squeeze are all issues the Government should have addressed years ago.
"Even with the extra money announced in the Budget, it still falls well short of what the NHS needs.
"It's incredibly difficult for the NHS to plan for a crisis when it is already running at capacity and staff are stretched all year round."
Labour's shadow health secretary Jonathan Ashworth said: "This serious warning from NHS Providers illustrates just how appalling this winter could be for patients.
"Serious questions remain around unsafe bed occupancy levels and the additional pressures a major flu outbreak would present on already overstretched services.
"Despite the Government announcing extra winter funding in the Budget, it beggars belief that this money has still not been allocated to struggling Trusts.
"The sad truth is that when this money does eventually arrive it will likely come far too late to be used to maximum effect."
A Department of Health spokesman said: "As NHS Providers say themselves in this report, trusts have done all they can to prepare for extra winter pressures this year.
"This has been supported by £435m to cope with winter, including making sure people get directed to the right service if they go to A&E, and £1bn this year to help meet adult social care needs.
"This year's winter preparation also includes an unprecedented system-wide push for all NHS workers to have the flu jab, aimed at helping protect patients in hospitals and in the community."