The casualty unit at Oxford's John Radcliffe Hospital is so over-stretched it is unsafe for patients, according to the department's top doctor.
Consultant Dr Philip Hormbrey said the volume of patients was clogging up the accident and emergency department, which was already understaffed.
He said the crisis was so bad that senior doctors and nurses were resigning.
Dr Hormbrey, the accident and emergency clinical director, has written to the General Medical Council saying the department is unsafe and he has warned doctors' leaders that it is not a safe place to train junior doctors.
He said: "We don't have the capacity to safely deal with the volume of patients coming through.
"On Monday, we had three times more patients than available cubicles and patients had to go on trolleys. In most cases, when the department is so overloaded, the clinical standards are inevitably going to suffer. Both doctors and nurses find it difficult to work. The safety of our patients comes first. My view is that if the system is at fault, someone must act. That was the view brought forward by my doctors who have resigned."
Dr Hormbrey's comments follow a report by the NHS South East Regional Office which criticised the way pressures at the JR casualty department were dealt with.
The problems have led to the resignations of two senior nurses, two senior house officers - due to stress-related illness - and a senior registrar, who said the department was dangerous.
Dr Hormbrey said: "We have the most fantastic team of nurses, but they have been under severe pressure for the past five years."
The JR's chief executive, David Highton, said: "We are aware that the emergency care system is under a great deal of pressure and that this is far from ideal.
"The emergency system in the county has little contingency within it at the moment."
Mr Highton said the hospital was working with other NHS organisations and social services on longer-term solutions.
A spokesman for the GMC said: "Dr Hormbrey has written to us and we will be investigating his concerns."
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