Some ambulances are being routed away from the Royal Sussex this morning because of IT issues.
Patients who are seriously ill or with major trauma and children are still being taken to the Brighton hospital, but lower priority patients are being taken to other hospitals.
The fault has also affected telephone lines, which means calls to the hospital may not be answered.
Outpatients are being asked to attend as normal, but warned there may be some disruption.
A spokesman for South East Coast Ambulance Service said: “We are aware of the issues and we are working closely with our hospital colleagues as they manage the current situation.
“We are diverting to other local hospitals for lower priority patients. We are still taking in seriously ill and major trauma patients as well as paediatric patients.
“We would ask the public, as ever, to consider all options available to them in seeking medical advice, including calling NHS 111 or speaking to their GP or a pharmacist.”
A statement on the Brighton and Sussex University Hospital Trust’s website says: “The trust is currently experiencing internal issues with our IT systems. This has also affected telephone lines. If you are trying to call us, your call may not be answered.
“If you have an outpatient appointment scheduled for today please attend as normal. There may be some disruption and we apologise for any inconvenience caused.”