The trust which runs the Princess Royal Hospital in Haywards Heath and the Royal Sussex County Hospital in Brighton, is urging people to stay away from A&E unless they need emergency treatment.
Brighton and Sussex University Hospitals Trust, which provides services for people who live across the Sussex Express area, has posted the message on the home page of its website.
It states: “We are currently under severe pressure - please don’t come to A&E unless you are in need of emergency treatment.
“Non-emergency patients are likely to have an extremely long wait and will be taking staff away from caring for patients who need emergency or life-saving care.
“Because of this severe pressure some appointments are being cancelled but we will call the patients affected directly. Please bear with us during this time of unprecedented demand and help us to prioritise patients who are in most urgent need.”
Sue Braysher, chief officer for NHS Horsham and Mid Sussex CCG said: “A&E and 999 are for emergencies - if it’s not an emergency, other services are available, 24 hours a day, 7 days a week. We know that one in three visits to A&E could be dealt with outside of hospital.”
The number of emergency calls being received is 30 per cent more than this time last year.