A hospital trust has declared a "critical incident" because of the "exceptional" pressure on A&E.
Nottingham University Hospitals Trust (NUH) runs the Queen's Medical Centre (QMC) and City Hospital and has been on OPEL 4 - previously known as black alert - since Monday morning.
On Wednesday it raised the level further.
Some routine operations have been cancelled as the trust prioritises those who need emergency care.
Health bosses do not want to operate on patients who cannot be guaranteed a bed in which to recover.
Lisa Kelly, NUH chief operating officer, said: "This is following a number of days seeing exceptional pressure across the system, with high numbers of very poorly patients arriving at our emergency department."
She added they were working closely with health and social care partners and trying to discharge patients in a "timely" manner to free up beds.
Mark Simmonds, the emergency department's clinical director, said at one point on Monday 14 ambulances arrived at the QMC in 15 minutes while they already had 160 patients in the department.
A spokesman for the trust explained that although they were seeing slightly more patients than might be expected, the main problem was those people coming in were more seriously ill than normal; in some cases these were flu patients.
The trust has been on OPEL 4 at least once this year but this is the first time in 2019 the pressure in the emergency department has been escalated to a critical incident.
The trust spokesman said it meant it could "focus energies on managing the pressures and bring health and social partners in to help support that, particularly with discharges".
Ms Kelly added: "This is not unique to Nottingham, and hospitals across the country are also experiencing similar pressures."
In the East Midlands, University Hospitals of Leicester and Sherwood Forest Hospitals NHS Foundation Trust were both on OPEL 4 - which means patient safety could be compromised - earlier this week.
They have since been scaled down to OPEL 3.