NHS Providers, the trade association that represents hospital, mental health, community and ambulance service trusts in England, called for an emergency cash injection of between Â£200 and Â£350 million to enable the NHS to manage patient safety risk in the winter months.
The organisation said failure to make the investment will lead to longer waiting times for patients in A&E and other services, and will also put the safety of patients at risk as local trusts have "insufficient capacity" to meet extra expected demand.
A report by NHS Providers has found that the level of planning and support for this winter - led jointly by NHS England and NHS Improvement - is considerably more developed than last year and emergency care performance has been given greater priority.
The report found extra social care funding is helping to increase capacity in about a third of local areas, and this should help to reduce the delays faced by some patients in those areas when they are medically fit to leave hospital but are unable to do so because of a lack of available support in the community.
But the trade association said these improvements are being outweighed by a combination of increasing risks.
It said NHS trusts are not consistently benefiting from the extra Â£1 billion of social care investment announced in the spring Budget, as planned, and as a result, delayed transfers of care for patients remain "stubbornly high".
It added that demand for emergency care is continuing its "inexorable rise", key staff shortages are growing, and primary and social care capacity, as a whole, remains "very challenged".
NHS Providers also said trusts are under greater financial pressure than last year and therefore less able to afford the extra capacity they urgently need.
Chris Hopson, chief executive of NHS Providers, said: "Last winter the health service came under pressure as never before.
"At its height, the NHS had to provide 4,500 additional beds a day - equivalent to more than eight extra hospitals.
"Patient safety was compromised as local services struggled to cope with the pressures.
"At times, in some places, the NHS was overwhelmed. We must act now to prevent the situation becoming even worse this winter.
"Trusts are doing all they can to prepare for this winter in the face of increasing demand for their services and competing priorities.
"And they are benefiting from much better national level planning from NHS England and NHS Improvement which is helping to identify and support those local areas that are most at risk.
"But despite this, the overwhelming view of NHS trusts is that without immediate extra funding they will not have sufficient capacity to manage this winter safely.
"This risk has been heightened because, in many areas, the Â£1 billion of extra support for social care announced in the Budget will not ease winter pressures on the NHS, as the Government had planned.
"Patients will therefore be put at greater risk as local trusts won't have the extra beds, staff and services they need to meet the extra demand they will face.
"The only way to mitigate these risks is through an urgent NHS cash injection to ensure the NHS has the necessary capacity this winter."
Niall Dickson, chief executive of the NHS Confederation, which represents health organisations in England, Wales and Northern Ireland, said: "This sombre report reflects the enormous pressure currently being faced by the NHS in England.
"As it makes clear, NHS trusts are likely to struggle this winter - the truth is that the whole system, including those who commission care, as well as those who provide both health and care services are facing very difficult choices.
"Last year the NHS managed incredibly well but we cannot continue just to rely on a hope that viruses will not wreak havoc, that the weather will be clement and that staff commitment will get us through.
"We will continue to push the Government for a comprehensive review looking at which services are needed, where they are needed, how much they will cost, and how they will be funded.
"As summer fades, the prospect of another difficult winter looms.
"The challenge lies not just in hospitals - we have shortages of community nurses, GPs, social care services and nursing home places, all of which are vital in taking pressure off the hospitals."
Pauline Philip, NHS national director for urgent and emergency care, said: "The NHS will face challenges this winter, as it does every year, but as NHS Providers have stated winter planning is more advanced than it was last year and, as they argue, special attention is being paid to areas where pressures are likely to be greatest.
"We are currently in the process of assessing how many extra beds trusts are planning to open over winter and early returns indicate that this will be more than 3,000.
"This is something we will continue to review on the basis of evidence rather than arbitrary estimates.
"If the expectations for reduced Delays Transfers of Care outlined by the Government are achieved, this would free up a further 2,000-3,000 beds over the winter period, on top of the extra 3,000-plus beds that hospitals now say they're going to open."
Shadow health secretary Jonathan Ashworth MP said: "Reports of 'dangerous' bed shortages this winter putting patient safety at risk are extremely worrying and will lead to yet more patients being left in the back of ambulances or stranded on hospital trolleys for hours on end.
"At the general election, Labour promised a cash injection to cope with this year's winter crisis.
"The Government urgently needs to follow our lead and to heed the advice from health bosses to provide an immediate funding boost this winter to avoid serious harm to patients."