New figures show patients are waiting an average of more than nine hours in Derriford Hospital's emergency department - with some forced to endure waits of 12 hours or longer.
Data from August, presented to the hospital trust's board, reveals that patients waited an average of 9 hours 22 minutes in ED to be either admitted, treated or discharged.
The NHS target states that 95 per cent of patients should be admitted, treated or discharged within four hours.
A spokesperson for the hospital said the entire healthcare system is under "extreme pressure" - but work is underway to reduce wait times.
A senior doctor who chairs the Royal College of Emergency Medicine in the region told ITV News the situation is 'extremely frightening' and 'not the way healthcare should be delivered'.
In August, 25% of patients attending the Emergency Department waited more than 12 hours.
A total of 778 patients spent more than 12 hrs waiting from being admitted to being given a bed - this is known as a 12-hour trolley wait.
On average patients waited for 37 minutes in ED for initial assessment. This has improved since March, when the average was 59 minutes, but is still more than twice the target of 15 minutes.
The figures also show that in August 72 per cent of ambulance handovers were delayed by more than 30 mins, meaning a total of 6,988 hrs hours lost to handover delays.
Dr James Gagg, South West vice-chair of the Royal College of Emergency Medicine, said: "The big issue is the amount of time ambulances are having to spend queued up outside our emergency departments trying to offload patients.
"This is a really significant problem, because we need those ambulances back on the road to meet the next 999 call. But emergency departments are often running at capacity with no further space for ambulances to offload."
Dr Gagg added: "Although I may be a doctor in the region, I am also a person who lives in the region. It might be that myself or my family need the response at some point. It's incredibly frightening. The level of concern in the industry is incredibly high. This is not right, it's not the way healthcare should be delivered."
A spokesperson from University Hospitals Plymouth NHS Trust said: “As a major trauma centre, some of our most complex patients arrive by road or air ambulance. Our staff are working as hard as they can to get patients into the emergency department and nobody wants to see patients waiting in ambulances.
“We have a lot of work going on within the hospital to allow us to get patients into our emergency department quicker. We have opened more beds to allow us to move patients from assessment units into wards and we are also moving patients into the discharge lounge throughout the day to get them ready to leave hospital.
“The local health and social care system is under extreme pressure. However, we are all working together to ensure patient safety, support each other and balance the demands of the wider health and social care system.”