The number of hospital trusts that are failing to meet A&E waiting time targets has doubled in a year, health regulators have said.
From October to December last year, 32 NHS foundation trusts failed to meet the four-hour target.
But for the same three-month period the previous year, the figure stood at 14, said regulator Monitor.
Hospitals aim to treat and either admit or discharge 95% of emergency cases within four hours.
Many who were unable to meet the target blamed outbreaks of the winter vomiting bug, discharge delays due to problems accessing community care services and increased attendances among elderly people. Monitor said it was "concerned" about the increasing number of trusts failing to meet waiting times.
Stephen Hay, managing director of provider regulation at Monitor, said: "There is a greater demand for A&E service across the NHS, but it is not acceptable that patients have to wait longer. Trusts need to work with local healthcare partners to understand and address these issues."
A spokesman said the regulator was also worried that only one in four foundation trusts is delivering the cost savings they said they would make this financial year.
The NHS is in the middle of a "productivity drive", with providers across the sector aiming to make £20 billion in efficiency savings by 2015.
Dr Peter Carter, chief executive and general secretary of the Royal College of Nursing, said: "This report is an unsettling but unsurprising insight into a system which is struggling to cope with fewer resources and rising demand, and it is patients who are suffering as a result.
"We know from our own research and from our members that the system is running at capacity, and cannot handle any extra demand such as winter viruses. It is unacceptable for patients to have to wait so long during an already difficult time."
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