Waiting times for accident and emergency patients have reached a nine-year high - with more patients having to endure a four-hour wait.
The monitoring report from The King's Fund showed that in the final quarter of 2012/13, 5.9% of patients (313,000 people) waited for four hours or longer in A&E, the highest level since 2004.
It is an increase of more than a third on the previous three months, and of nearly 40% since the same quarter the previous year.
It means that the Government's target that no more than 5% of patients should wait for more than four hours has been broken for the first time since June 2011, when they promised to keep waiting levels low.
John Appleby, chief economist at The King's Fund, said: "Emergency care acts as a barometer for the NHS.
"The worryingly high number of patients waiting longer than four hours in the last quarter of 2012/13 is a clear warning sign that the health system is under severe strain.
"The pressures in emergency care will not be relieved by focusing on a single aspect of the problem in isolation - it requires a co-ordinated response across the whole health system.
'While the NHS is in a healthy financial position overall, efficiencies are becoming harder to deliver as one-off savings
such as cuts in management costs start to slow. This is compounded by the need to maintain staffing levels following the shocking failures of care highlighted by the Francis report.
"With staff costs making up the bulk of the NHS budget, this will leave little room for manoeuvre - significant changes to services will be required if the NHS is to meet its target of delivering £20 billion in efficiency savings."
Nearly 40% of trusts reported that they breached the waiting time target in the last quarter. The King's Fund also said that the proportion of patients waiting more than four hours before being admitted to hospital from A&E has risen to nearly 7% - again, the highest level since 2004.
The charity said the analysis shows the "severe strain on emergency care in early 2013" and that there is a risk the same thing could happen next winter.
However, a spokeswoman for The King's Fund said: "Despite the pressures in emergency care, other NHS performance measures are continuing to hold up well. Waiting times for referral to treatment in hospital, the number of health care-acquired infections and delays in transferring patients out of hospital all remain stable."
A survey of NHS finance directors also conducted by The King's Fund found that the NHS is set to end the 2012/13 period in a healthy financial position, but that the outlook for the following two years is bleak. Most expect the NHS to fail to meet its target of delivering £20 billion in productivity improvements by 2015, The King's Fund said.
Andy Burnham, Labour’s Shadow Health Secretary, said: "The revelation that A&E waiting times have hit a ten-year high under David Cameron demolishes once and for all the Tory spin that the A&E crisis has nothing to do with them.
"David Cameron and Jeremy Hunt have wasted precious time provoking a fight with GPs to distract attention from their own failings.
"Their arrogance and complacency in the face of a crisis is one of the greatest dangers the NHS faces."
A Department of Health spokesman said: "Overall the NHS is performing well and for the fourth week in a row 95% of A&E patients have been seen within four hours."
She added: "Clearly the NHS had a difficult winter and A&E departments were under pressure. This is partly due to the fact that there are over one million more people visiting A&E compared to three years ago.
"We've been absolutely clear that the NHS needs to transform the way health services are delivered to meet the needs of an ageing population focusing more on joined up services and prevention."Suggest a correction