The NHS faces "severe pressure" this winter with an increase in demand for services related to flu, norovirus, and respiratory conditions, health service experts have warned.
NHS Providers, a trade organisation for NHS health workers, said the service was "not where it would want to be" heading into the colder months, and that occupancy rates for beds are already running above the recommended safety levels.
In a briefing paper, NHS Providers said trusts were preparing for winter but there are still "risks" in the system, including a more virulent flu strain, problems with capacity, workforce shortages and pressurised finances.
The paper further warned that this year's flu strain was "potentially the worst we have seen in two decades".
Other problems included shortages of key staff groups including paramedics, GPs and A&E consultants and nurses.
- Life inside A&E as staff prepare for winter:
NHS Providers said some steps had been taken to mitigate winter problems, including more money to deal with seasonal pressures.
However, Chris Hopson, its chief executive, 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 cannot 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."
Dr Nick Scriven, president of the Society for Acute Medicine, said: "While staff on the front line 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."
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."
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."