The NHS is facing a "toxic mix" as winter approaches of deficits, rising waiting times and low staff morale, an influential think tank has warned.
Cuts to social care are also damaging the NHS and creating problems with so-called bed-blocking not seen for almost a decade, the King's Fund said.
In a new report, it said managers believed tougher new rules to cap the spend on agency staff would affect how hospitals performed.
Just over a quarter (27%) of NHS trust finance directors surveyed by the King's Fund said the cap would affect their ability to ensure safe staffing levels.
This comes just as the NHS enters the winter period, "with its highest demands for staff", the report said.
As winter approaches, the total number on waiting lists also "continues to rise and now, at nearly 3.5 million, is at its highest for more than seven years".
The four-hour target for patients to be seen in A&E remains a "challenge" and has been met across the NHS only once in the past 12 months.
Meanwhile, the number of patients waiting more than four hours for admission to a bed from emergency departments - so-called trolley waits - stood at around 20,000 for August, 25% higher for this month than in the past four years.
"This does not augur well for winter and early spring, when demand pressures will mount," the report warned.
The King's Fund found that 88% of NHS trust finance directors believed cuts to social care were impacting on the NHS.
Some 80% of clinical commissioning group finance directors, who oversee groups of GPs, also believed the NHS was suffering due to social care cuts.
The quarterly monitoring report included analysis of how many people were forced to stay in hospital because services were not in place for them to move back home or into care homes.
More than 5,000 patients experienced delays in being discharged from hospital at the end of August - the highest level at this time of year since 2007, the report said.
Almost a third of these delays were caused by problems accessing social care services - an increase of 21% in the past year.
The King's Fund said it was calling on the government to use the forthcoming Spending Review to protect social care from further budget cuts and reinvest £6 billion previously set aside.
The report also found that the vast majority of NHS trusts were forecasting a deficit at the end of the financial year.
Reports from regulators Monitor and the Trust Development Authority have shown that trusts overspent by £930 million in the first three months of the financial year.
John Appleby, chief economist at the King's Fund, said: "The quarterly monitoring report reveals the financial crisis engulfing the NHS and social care.
"With winter approaching, the NHS faces a toxic mix of widespread deficits, rising waiting times and low morale.
"There is now clear evidence that cuts to social care budgets are affecting the NHS, as well as reducing services for people that need them."
Dr Ian Wilson, from the British Medical Association (BMA), said: "The NHS is facing a funding crisis the likes of which we have never seen, and despite politicians' promises, current funding is barely enough for the health service to stand still.
"With winter just around the corner, which will undoubtedly increase pressures on services and staff, it is also deeply worrying that trust finance directors continue to raise concerns over staff morale. The Government must wake up and take action."
Janet Davies, chief executive of the Royal College of Nursing (RCN), said: "The NHS does not operate in a vacuum and when money is taken away from public health and social care budgets, it becomes harder to prevent hospital admissions, and harder to discharge patients promptly.
"Cuts to social services make it harder for people to access essential support to allow them to live independently. This is devastating for individuals, and puts even more pressure on already over-stretched health services."