One in six accident and emergency departments in England could be shut or downgraded in the coming years, according to a new analysis.
There are 24 A&Es on a list drawn up by researchers at the Health Service Journal (HSJ).
The report comes weeks after warnings that the health service was in the grip of a "humanitarian crisis", with some emergency departments facing such high demand they had to turn patients away.
NHS officials said some changes to A&Es had been planned and statistics showed treatment could be improved with "concentration" of specialist services.
However the health service said it does not expect significant changes to A&Es in the years ahead.
The HSJ analysis said 15% of emergency departments could be "closed or downgraded" in the next four years.
Dr Chris Moulton, vice president of the Royal College of Emergency Medicine, warned the NHS does not have enough capacity for a growing and ageing population.
"Any A&E closures must be very carefully considered for patient safety, patient convenience and the effects on neighbouring departments that would have to absorb the extra patient attendances," he said.
Liberal Democrat health spokesman Norman Lamb raised concerns that the plans were going ahead without the public being consulted.
"People won't put up with the destruction of local services imposed from on high," he said.
In January the Red Cross warned the NHS was facing a "humanitarian crisis", with elderly and vulnerable patients stranded in desperately needed beds in A&Es due to a lack of social care.
Two people died after spending hours in the corridor of one A&E department, while other hospitals were forced to send patients elsewhere because they had no space.
The NHS said the range of services available to patients is expected to expand as the number of people seeking urgent care increases over the coming years.
A spokesman said: "Within that overall expansion, it may be possible to improve care and save lives with some concentration of specialist urgent services.
"This approach has increased the chances of surviving a major trauma in this country by 50%, and only today the Stroke Association have called for more concentration of stroke units to improve outcomes."
"However we do not expect significant numbers of A&E changes in the years ahead."