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 an overall rating of "inadequate".
Portsmouth Hospitals NHS Trust's chief executive resigned in May.
The Trust gave no reason for Ursula Ward's decision to stand down.
CQC inspectors, who visited in February and March, said the emergency department was "chaotic" with an "unacceptable" risk to patients.
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 the country 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 5 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.
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."

Liverpool made 11 changes and Tottenham 10 on Tuesday and the 10 top-flight teams made 71 changes, with a number of young or fringe players involved.
"I feel for the fans," said 55-year-old Waddle. "They should know who is going to play in the competition."
Former Tottenham manager David Pleat also said he was "very disappointed".
Pleat, who reached the semi-finals of the competition in 1987 as manager of Tottenham, said teams are treating the competition as a "secondary affair".
"It is almost as if the Premier League clubs have got together and said we are not going to give the competition our full attention," he said.
Ex-Spurs and Newcastle player Waddle said clubs should announce their squad for the cup's fixtures in advance at the start of the week.
"It would mean that if you're travelling a long way you'd know that it will be a weaker side and you may not buy a ticket," he told BBC Radio 5 live.
"For me, it is a total rip-off."
Tottenham manager Mauricio Pochettino, whose side are fifth in the Premier League and second in their Champions League group, defended his decision to use some of the younger members of his squad in Tuesday's 2-1 defeat at Anfield.
"The young players are very important for our future, to be a better squad," he said.
"I think it was a fantastic opportunity to show their quality and a great experience to capitalise on, to take positive things for the future."
Health inspectors have condemned a "chaotic" hospital emergency department which they said was failing to keep patients safe.
[NEXT_CONCEPT]
The EFL Cup is a "total rip-off" for fans because Premier League managers field weakened teams, says former England winger Chris Waddle.