Doctors at Norfolk's biggest hospital have been told to make the "least unsafe decision" to try and cope with what it has called "extreme pressure".
The advice was sent to staff at the Norfolk and Norwich University Hospital on Tuesday in what it described as a "hastily-worded email".
It came as the A&E department reached capacity with a huge rise in admissions.
MPs have called for emergency funding and more support for the hospital.
Erika Denton, NNUH Medical Director, said: "The hospital was under extreme pressure earlier this week with very high emergency department attendances and a rise in admissions."
She said the email to doctors "had a number of actions to help address the challenges we were facing".
"The Trust is resolute in its determination to provide timely, safe, high quality care to all our patients and our staff are diligently working to ensure this happens."
The hospital also said it was "more important than ever" for the public to help NHS staff by getting flu jabs.
They asked people with Norovirus to follow advice on the NHS website and to use the NHS 111 phone or online service for advice on urgent medical needs.