The NHS has made an urgent appeal to people in Greater Manchester to only go to A&E departments if they have "a life-threatening emergency".
High numbers of patients and staff sickness has led to pressure "beyond anything we have experienced before", NHS Greater Manchester Integrated Care said.
An increase in flu and Covid
was also blamed for "very long waits".
People are asked to use other services, including NHS 111, GPs and pharmacists.
Prof Jane Eddleston, Greater Manchester Medical Executive lead for acute care, said it was always busy after the Christmas break but this "has been beyond anything we have experienced before".
"We are making an urgent plea to the public in Greater Manchester to only call 999 or attend Accident and Emergency Departments if their condition is a life-threatening emergency.
"Unprecedented levels of attendance at A&E, staff sickness and increased prevalence of flu and Covid
have led to very long waits in emergency departments and for ambulances."
GP practices and pharmacies are open as normal between Christmas and New Year, the NHS added.