Some planned procedures and appointments at Nottingham University Hospitals Trust are expected to be cancelled today due to an IT issue.
The trust says it's significantly impacted their ability to process and report on blood samples, slowing down clinical decision making.
This has caused delays in discharging people and resulting in long ambulance waits and overcrowding in the Emergency Department.
It says additional measure are being taken to maintain safe services.
The trust has apologised for the inconvenience and say they will contact those affected directly.
It said the emergency department will remain open, but ask people only to attend if they have a life-threatening condition or injury.