Derriford Hospital's medical director says staff are 'extremely busy' with 'long delays' in the emergency department.
Dr Phil Hughes is urging people not to visit the hospital unless absolutely necessary.
On Tuesday afternoon the longest waiting time to see a doctor in the emergency department was more than three hours, with more than 60 patients waiting for treatment.
Alternatives to A&E
Patients are instead advised to consider visiting a minor injury unit, a pharmacist or their local GP.
The NHS’ 111 number is also available for people who urgently need medical help or advice but are not in a life-threatening situation.