The Royal Sussex County Hospital in Brighton has declared a critical incident because it is full.
The hospital is trying to discharge as many patients as possible so they can continue to admit emergencies.
Meetings have been cancelled so staff can work on wards, extra clinicians have been dispatched to work in outpatients and A&E staff brought in to assess patients.
We spoke to Dr George Findlay Chief Medical Officer.
South East Coast Ambulance Service said it couldn't take patients to other hospitals because all nearby sites in Sussex faced equally busy sites.