Health inspectors have condemned a "chaotic" hospital emergency department which they said was failing to keep patients safe.
The Care Quality Commission (CQC) found long queues of ambulances outside the Queen Alexandra Hospital in Portsmouth, while patients with serious symptoms waited hours to be assessed.
It gave the hospital's A&E service a rating of "inadequate".
Interim chief executive Tim Powell has apologised and pledged to improve.
In May, Portsmouth Hospitals NHS Trust's then chief executive Ursula Ward resigned but no reason was given.
CQC inspectors who visited in February and March, said the emergency department was "chaotic" with an "unacceptable" risk to patients.
As an overall rating, however, the hospital was told it "requires improvement".
At times, sixteen ambulances were stacking up outside the department, representing a third of the emergency ambulance fleet for Hampshire.
Inspectors found on 11 occasions, there were no ambulances available to respond to 999 calls.
Vulnerable patients including frail pensioners were left in ambulances for more than two hours.
The department was "severely overcrowded" with some patients waiting on trolleys, in corridors, or in a specialised "Jumbulance", a large ambulance, parked outside.
Waiting times were among the worst in England with almost a third of patients waiting more than four hours to be treated.
Patients with suspected life-threatening conditions such as stroke or sepsis were not assessed quickly, and one patient with potential heart failure waited more than five hours for a first assessment.
The CQC said the trust had been slow to improve despite warnings and had to take "immediate action" to protect patients, including ceasing the use of the Jumbulance.
'We are very sorry'
The Conservative MP for Portsmouth South, Flick Drummond, said: "I was horrified to read in the report that CQC inspectors were having to look after people themselves during their visit."
She said the hospital had told her it was bringing in a specialist in acute medicine from a neighbouring NHS Trust, who would start work in July.
Another Hampshire Conservative MP, George Hollingbery, who represents Meon Valley, said: "I'm heartened the trust has apologised for its failings... but, sadly, we have been here before and patience is understandably wearing thin."
Mark Ainsworth, of South Central Ambulance Service, said there had been "significant improvements" in the flow of patients into the emergency department since the CQC inspection was carried out.
He said: "It's disappointing it's taken that CQC report to see that change happen, but it has been the push factor that has led to the changes."
David Cozens, of the Royal College of Nursing (RCN), said the report was "very grim reading but it does not come as a surprise".
"We know that nursing staff are working under extreme pressures, battling to provide the best possible care in an incredibly challenging environment.
"The trust has implemented changes to management and has action plans in place but, as the CQC highlights, it cannot solve this problem on its own and will need support from other local health and social care services," he added.
The NHS trust's interim Chief Executive, Tim Powell, said: "We recognise the picture painted by the CQC in this report and we are very sorry that we have failed to provide to our patients, on a consistent basis, the high standards of care that we expect of ourselves.
"We fully accept the inspector's findings and... we are determined to ensure that by... next winter, our service will be better."