The number of patients facing crippling waiting times in England's accident and emergency units has reached an eight-year peak, according to a new a study.
NHS performance data revealed that the proportion of people facing waits of more than four hours for treatment has increased by more than a quarter, reaching its highest level since 2004.
The King's Fund report showed that 4.2% of A&E patients waited longer than four hours from January to March, compared with 3.4% in the same period last year.
Its authors said the increase was a cause for concern and reflected growing pressures on the hospital sector.
On a national level the government's target that no more than 5% of patients face more than a four-hour wait in A&E was met last year, the think-tank's quarterly monitoring report found.
But 48 NHS providers breached the threshold in the final quarter of last year compared with 18 in the second quarter.
The King's Fund said the rise coincides with emerging evidence of increases in "trolley waits" as some hospitals struggle to find beds for patients.
Data obtained from 60 NHS finance directors as part of the study also revealed that 40% of trusts did not meet their productivity targets in 2011/12.
The report said: "This will be a significant concern as last year was the first in a four-year spending squeeze, during which the NHS needs to find £20 billion in productivity improvements."
Just four of the finance chiefs questioned said their organisation was forecasting a deficit this year, backing up national figures that estimate a surplus of £1.5 billion across the NHS.
The study found the NHS was performing well against a number of other key indicators including hospital treatment waiting times and superbug infections.
The proportion of inpatients waiting more than 18 weeks for treatment fell, while outpatient waits remained static, the report said. C difficile and MRSA infections dropped by 33% and 14% respectively.
John Appleby, chief economist at the King's Fund, said: "Overall, the NHS continues to perform well, despite the spending squeeze. However, this masks growing pressures in hospitals and significant performance issues in some NHS organisations.
"Given the strength of the political commitment to keep waiting times low, the steep rise in A&E waits will be a concern for the government.
"The productivity challenge will only get harder, so evidence that large numbers of NHS organisations failed to meet their productivity targets last year does not bode well."
However Health Secretary Andrew Lansley has lambasted the results, questioning the way the report measured time spent in A&E:
"This measure records the total time people spend in the department, including the time they are being treated; it does not just measure waiting" he said.
"The King's Fund report is wrong to suggest the proportion of people 'waiting' more than four hours to be seen in A&E is growing.
"People are waiting on average only 49 minutes for their treatment to start. In addition, the NHS has continued to meet or exceed the target of 95% of patients spending a total time of four hours or less in A&E."